[media] v4l: v4l2-ctrls: moves the forward declaration of struct file

This fixes the following warning:

In file included from drivers/media/video/v4l2-subdev.c:29:
include/media/v4l2-ctrls.h:501: warning: 'struct file' declared inside
parameter list
include/media/v4l2-ctrls.h:501: warning: its scope is only this
definition or declaration, which is probably not what you want
include/media/v4l2-ctrls.h:509: warning: 'struct file' declared inside
parameter list

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>