[media] v4l2-fh: Include linux/videodev2.h for enum v4l2_priority definition

struct v4l2_fh has an enum v4l2_priority field. Make sure the enum
definition is available by including linux/videodev2.h.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>