libcamera: camera_manager: Make CameraManager::Private::mutex_ mutable

The mutex_ stored in the CameraManager::Private class is used to protect
members that may need to be accessed from const functions. Make it
mutable to allow this.

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

src/libcamera/camera_manager.cpp

@@ -50,7 +50,7 @@ public:
 	 * - initialized_ and status_ during initialization
 	 * - cameras_ and camerasByDevnum_ after initialization
 	 */
-	Mutex mutex_;
+	mutable Mutex mutex_;
 	std::vector<std::shared_ptr<Camera>> cameras_;
 	std::map<dev_t, std::weak_ptr<Camera>> camerasByDevnum_;