device-monitor: Deinitialize GStreamer before exiting

Nice to have when using the leak tracer.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/8670>

subprojects/gst-plugins-base/tools/gst-device-monitor.c

@@ -412,6 +412,7 @@ real_main (int argc, char **argv)
   g_main_loop_unref (app.loop);
   g_timer_destroy (timer);
 
+  gst_deinit ();
   return 0;
 }