The V4L2Subdevice class uses MediaEntity instances, and the
corresponding header is not included.
Fix this by forward declaring the MediaEntity class.
Fixes: 468176fa07 ("libcamera: Add V4L2Subdevice")
Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>