[media] Fix build when drivers are builtin and frontend modules

There are a large number of reports that the media build is
not compiling when some drivers are compiled as builtin, while
the needed frontends are compiled as module.

On the last one of such reports:
	From: kbuild test robot <fengguang.wu@intel.com>
	Subject: saa7134-dvb.c:undefined reference to `zl10039_attach'

The .config file has:

	CONFIG_VIDEO_SAA7134=y
	CONFIG_VIDEO_SAA7134_DVB=y
	# CONFIG_MEDIA_ATTACH is not set
	CONFIG_DVB_ZL10039=m

And it produces all those errors:

   drivers/built-in.o: In function `set_type':
   tuner-core.c:(.text+0x2f263e): undefined reference to `tea5767_attach'
   tuner-core.c:(.text+0x2f273e): undefined reference to `tda9887_attach'
   drivers/built-in.o: In function `tuner_probe':
   tuner-core.c:(.text+0x2f2d20): undefined reference to `tea5767_autodetection'
   drivers/built-in.o: In function `av7110_attach':
   av7110.c:(.text+0x330bda): undefined reference to `ves1x93_attach'
   av7110.c:(.text+0x330bf7): undefined reference to `stv0299_attach'
   av7110.c:(.text+0x330c63): undefined reference to `tda8083_attach'
   av7110.c:(.text+0x330d09): undefined reference to `ves1x93_attach'
   av7110.c:(.text+0x330d33): undefined reference to `tda8083_attach'
   av7110.c:(.text+0x330d5d): undefined reference to `stv0297_attach'
   av7110.c:(.text+0x330dbe): undefined reference to `stv0299_attach'
   drivers/built-in.o: In function `tuner_attach_dtt7520x':
   ngene-cards.c:(.text+0x3381cb): undefined reference to `dvb_pll_attach'
   drivers/built-in.o: In function `demod_attach_lg330x':
   ngene-cards.c:(.text+0x33828a): undefined reference to `lgdt330x_attach'
   drivers/built-in.o: In function `demod_attach_stv0900':
   ngene-cards.c:(.text+0x3383d5): undefined reference to `stv090x_attach'
   drivers/built-in.o: In function `cineS2_probe':
   ngene-cards.c:(.text+0x338b7f): undefined reference to `drxk_attach'
   drivers/built-in.o: In function `configure_tda827x_fe':
   saa7134-dvb.c:(.text+0x346ae7): undefined reference to `tda10046_attach'
   drivers/built-in.o: In function `dvb_init':
   saa7134-dvb.c:(.text+0x347283): undefined reference to `mt352_attach'
   saa7134-dvb.c:(.text+0x3472cd): undefined reference to `mt352_attach'
   saa7134-dvb.c:(.text+0x34731c): undefined reference to `tda10046_attach'
   saa7134-dvb.c:(.text+0x34733c): undefined reference to `tda10046_attach'
   saa7134-dvb.c:(.text+0x34735c): undefined reference to `tda10046_attach'
   saa7134-dvb.c:(.text+0x347378): undefined reference to `tda10046_attach'
   saa7134-dvb.c:(.text+0x3473db): undefined reference to `tda10046_attach'
   drivers/built-in.o:saa7134-dvb.c:(.text+0x347502): more undefined references to `tda10046_attach' follow
   drivers/built-in.o: In function `dvb_init':
   saa7134-dvb.c:(.text+0x347812): undefined reference to `mt352_attach'
   saa7134-dvb.c:(.text+0x347951): undefined reference to `mt312_attach'
   saa7134-dvb.c:(.text+0x3479a9): undefined reference to `mt312_attach'
>> saa7134-dvb.c:(.text+0x3479c1): undefined reference to `zl10039_attach'

This is happening because a builtin module can't use directly a symbol
found on a module. By enabling CONFIG_MEDIA_ATTACH, the configuration
becomes valid, as dvb_attach() macro loads the module if needed, making
the symbol available to the builtin module.

While this bug started to appear after the patches that use IS_DEFINED
macro (like changeset 7b34be71), this
bug is a way ancient than that.

The thing is that, before the IS_DEFINED() patches, the logic used to be:

       && defined(MODULE))
struct dvb_frontend *zl10039_attach(struct dvb_frontend *fe,
					u8 i2c_addr,
					struct i2c_adapter *i2c);
static inline struct dvb_frontend *zl10039_attach(struct dvb_frontend *fe,
					u8 i2c_addr,
					struct i2c_adapter *i2c)
{
	printk(KERN_WARNING "%s: driver disabled by Kconfig\n", __func__);
	return NULL;
}

The above code, with the .config file used, was evoluting to FALSE
(instead of TRUE as it should be, as CONFIG_DVB_ZL10039 is 'm'),
and were adding the static inline code at saa7134-dvb, instead
of the external call. So, while it weren't producing any compilation
error, the code weren't working either.

So, as the overhead for using CONFIG_MEDIA_ATTACH is minimal, just
enable it, if MODULES is defined.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>