USB: export video.h to the includes available for userspace

The uvcvideo extension unit API requires constants defined in the
video.h header. Add it to the list of includes exported to userspace.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>