[media] Fix build errors on usbtv when driver is builtin

As reported by Fengguang Wu <fengguang.wu@intel.com>

   drivers/built-in.o: In function `vb2_ioctl_streamon':
>> (.text+0x8d354): undefined reference to `video_devdata'
   drivers/built-in.o: In function `vb2_ioctl_streamoff':
>> (.text+0x8d397): undefined reference to `video_devdata'
   drivers/built-in.o: In function `vb2_ioctl_expbuf':
...

That happens when:
	CONFIG_VIDEO_DEV=y
	CONFIG_VIDEO_V4L2=m
	CONFIG_VIDEO_USBTV=y

As the core is module, usbtv should also be compiled as module.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>