V4L/DVB (10738): Get rid of video_decoder.h header were uneeded

Now, just a few modules are still dependent of this legacy header:

$ grep -l DECODER_ `find linux/drivers/media/ -name '*.[ch]' -exec grep -l video_decoder '{}' \;`
linux/drivers/media/video/v4l2-ioctl.c
linux/drivers/media/video/indycam.c
linux/drivers/media/video/saa7191.c
linux/drivers/media/video/vino.c

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