rtpsource: Skip some stats when GAP flag is set [pexhack]
rtpsource: Skip some stats when GAP flag is set [pexhack]

GST_BUFFER_FLAG_GAP indicates that the buffer has been created to fill a
gap in the stream and was not actually received.

pexip/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good