netconsole: fix build when CONFIG_NETCONSOLE_DYNAMIC is turned on

When NETCONSOLE_DYNAMIC=y and CONFIGFS_FS=m, there are build errors
in netconsole:

drivers/built-in.o: In function `drop_netconsole_target':
netconsole.c:(.text+0x1a100f): undefined reference to `config_item_put'
drivers/built-in.o: In function `make_netconsole_target':
netconsole.c:(.text+0x1a10b9): undefined reference to `config_item_init_type_name'
drivers/built-in.o: In function `write_msg':
netconsole.c:(.text+0x1a11a4): undefined reference to `config_item_get'
netconsole.c:(.text+0x1a1211): undefined reference to `config_item_put'
drivers/built-in.o: In function `netconsole_netdev_event':
netconsole.c:(.text+0x1a12cc): undefined reference to `config_item_put'
netconsole.c:(.text+0x1a12ec): undefined reference to `config_item_get'
netconsole.c:(.text+0x1a1366): undefined reference to `config_item_put'
drivers/built-in.o: In function `init_netconsole':
netconsole.c:(.init.text+0x953a): undefined reference to `config_group_init'
netconsole.c:(.init.text+0x9560): undefined reference to `configfs_register_subsystem'
drivers/built-in.o: In function `dynamic_netconsole_exit':
netconsole.c:(.exit.text+0x809): undefined reference to `configfs_unregister_subsystem'

so fix the NETCONSOLE_DYNAMIC depends clause to prevent this.
Based on email suggestion from Ben Hutchings.  Thanks.

Fixes https://bugzilla.kernel.org/show_bug.cgi?id=37992



Reported-by: David Hill <hilld@binarystorm.net>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>