Cameras can be dynamically added or removed on OS X. Make sure
the cache is updated often enough so QCameraInfo::availableCameras()
return an up to date list.
Task-number: QTBUG-39708
Change-Id: Id806d52278e1a29163fcc6707da7f86c0f3e7c0d
Reviewed-by: Timur Pocheptsov <Timur.Pocheptsov@digia.com>