[media] mt9p031.c included media/v4l2-subdev.h twice

drivers/media/video/mt9p031.c included 'media/v4l2-subdev.h' twice,
remove the duplicate.

Signed-off-by: Danny Kukawka <danny.kukawka@bisect.de>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>