modpost: detect modules without a MODULE_LICENSE

Partially revert commit 2fa36568 ("kbuild: soften MODULE_LICENSE
check") so that modpost detects modules that do not have a
MODULE_LICENSE.

Sam's commit also changed the fatal error to a warning, which I am
leaving as is.

This gives advance notice of when a module has no license and will taint
the kernel if the module is loaded.

This produces the following warnings on x86_64 allmodconfig:

    MODPOST 6520 modules
  WARNING: modpost: missing MODULE_LICENSE() in drivers/auxdisplay/img-ascii-lcd.o
  WARNING: modpost: missing MODULE_LICENSE() in drivers/gpio/gpio-ath79.o
  WARNING: modpost: missing MODULE_LICENSE() in drivers/gpio/gpio-iop.o
  WARNING: modpost: missing MODULE_LICENSE() in drivers/iio/accel/kxsd9-i2c.o
  WARNING: modpost: missing MODULE_LICENSE() in drivers/iio/adc/qcom-vadc-common.o
  WARNING: modpost: missing MODULE_LICENSE() in drivers/media/platform/mtk-vcodec/mtk-vcodec-common.o
  WARNING: modpost: missing MODULE_LICENSE() in drivers/media/platform/soc_camera/soc_scale_crop.o
  WARNING: modpost: missing MODULE_LICENSE() in drivers/mtd/nand/denali_pci.o
  WARNING: modpost: missing MODULE_LICENSE() in drivers/net/phy/cortina.o
  WARNING: modpost: missing MODULE_LICENSE() in drivers/pinctrl/pxa/pinctrl-pxa2xx.o
  WARNING: modpost: missing MODULE_LICENSE() in drivers/power/reset/zx-reboot.o
  WARNING: modpost: missing MODULE_LICENSE() in drivers/rpmsg/qcom_glink_native.o
  WARNING: modpost: missing MODULE_LICENSE() in drivers/staging/comedi/drivers/ni_atmio.o
  WARNING: modpost: missing MODULE_LICENSE() in net/9p/9pnet_xen.o
  WARNING: modpost: missing MODULE_LICENSE() in sound/soc/codecs/snd-soc-pcm512x-spi.o

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>