[media] v4l2-mc.h: fix compiler warnings

[media] v4l2-mc.h: fix compiler warnings

Fix these warnings when CONFIG_MEDIA_CONTROLLER is not defined:

In file included from drivers/media/v4l2-core/v4l2-fh.c:32:0:
include/media/v4l2-mc.h:173:12: warning: 'v4l_enable_media_source' defined but not used [-Wunused-function]
 static int v4l_enable_media_source(struct video_device *vdev)
            ^
include/media/v4l2-mc.h:183:12: warning: 'v4l_vb2q_enable_media_source' defined but not used [-Wunused-function]
 static int v4l_vb2q_enable_media_source(struct vb2_queue *q)
            ^
In file included from include/media/tuner.h:23:0,
                 from drivers/media/tuners/tuner-types.c:9:
include/media/v4l2-mc.h:173:12: warning: 'v4l_enable_media_source' defined but not used [-Wunused-function]
 static int v4l_enable_media_source(struct video_device *vdev)
            ^
include/media/v4l2-mc.h:178:13: warning: 'v4l_disable_media_source' defined but not used [-Wunused-function]
 static void v4l_disable_media_source(struct video_device *vdev)
             ^

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>