libcamera: media_object: Add a const version of dev()

Add a const version of the MediaObject::dev() method to be able to
retrieve a pointer to a const MediaDevice from a constant instance of
a MediaObject sub-class.

Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>

include/libcamera/internal/media_object.h

@@ -22,6 +22,7 @@ class MediaObject
 {
 public:
 	MediaDevice *device() { return dev_; }
+	const MediaDevice *device() const { return dev_; }
 	unsigned int id() const { return id_; }
 
 protected: