macOS: fix add video device

Add device should be called after initializing video device monitor.

Change-Id: I2f4dda929086837ccc1fb1f854b6827d203092d8

src/media/video/osxvideo/video_device_monitor_impl.mm

@@ -62,6 +62,10 @@ class VideoDeviceMonitorImpl {
 
 VideoDeviceMonitorImpl::VideoDeviceMonitorImpl(VideoDeviceMonitor* monitor) :
     monitor_(monitor)
+{
+}
+
+void VideoDeviceMonitorImpl::start()
 {
     /* Enumerate existing devices */
     auto myVideoDevices = [[AVCaptureDevice devicesWithMediaType:AVMediaTypeVideo]
@@ -88,11 +92,7 @@ VideoDeviceMonitorImpl::VideoDeviceMonitorImpl(VideoDeviceMonitor* monitor) :
             RING_ERR("%s", e.what());
         }
     }
-    return;
-}
     
-void VideoDeviceMonitorImpl::start()
-{
     NSNotificationCenter *notificationCenter = [NSNotificationCenter defaultCenter];
     id deviceWasConnectedObserver = [notificationCenter addObserverForName:AVCaptureDeviceWasConnectedNotification
                                     object:nil