GStreamer: fix memory leaks.

Many GStreamer objects were not properly managed or never released.

Change-Id: I38b3854e8b9e2264b5b647f331d3bb16b886e2d6
Reviewed-by: Andrew den Exter <andrew.den.exter@qinetic.com.au>

src/plugins/gstreamer/audiodecoder/qgstreameraudiodecoderserviceplugin.cpp

@@ -147,6 +147,7 @@ void QGstreamerAudioDecoderServicePlugin::updateSupportedMimeTypes() const
                                         }
                                     }
                                 }
+                                gst_caps_unref(caps);
                             }
                         }
                         gst_object_unref (factory);