[media] V4L: fix a compiler warning

Fix the warning:

In file included from /home/lyakh/software/project/24/src/linux-2.6/drivers/media/video/v4l2-subdev.c:29:
linux-2.6/include/media/v4l2-ctrls.h:497: warning: 'struct file' declared inside parameter list
linux-2.6/include/media/v4l2-ctrls.h:497: warning: its scope is only this definition or declaration, which is probably not what you want
linux-2.6/include/media/v4l2-ctrls.h:505: warning: 'struct file' declared inside parameter list
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

include/media/v4l2-ctrls.h

@@ -496,6 +496,7 @@ void v4l2_ctrl_add_event(struct v4l2_ctrl *ctrl,
 void v4l2_ctrl_del_event(struct v4l2_ctrl *ctrl,
 		struct v4l2_subscribed_event *sev);
 
+struct file;
 /* Can be used as a vidioc_log_status function that just dumps all controls
    associated with the filehandle. */
 int v4l2_ctrl_log_status(struct file *file, void *fh);