aggregator: allow src GstAggregatorPads
aggregator: allow src GstAggregatorPads

See https://bugzilla.gnome.org/show_bug.cgi?id=793917

https://bugzilla.gnome.org/show_bug.cgi?id=793934

StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer