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/multimedia/gsttools_headers/qgstutils_p.h

@@ -78,6 +78,8 @@ namespace QGstUtils {
                                              const QSet<QString> &supportedMimeTypeSet);
 }
 
+void qt_gst_object_ref_sink(gpointer object);
+
 QT_END_NAMESPACE
 
 #endif