aggregator: use new gst_element_foreach_sink_pad()
aggregator: use new gst_element_foreach_sink_pad()

Instead of gst_aggregator_iterate_sinkpads() which will
soon be removed.

https://bugzilla.gnome.org/show_bug.cgi?id=785679

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