Preliminary gstreamer 1.20 compatibility

The transceiver was made private in
https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/2241

The direction property was added here:
28b0be4036

src/voip/WebRTCSession.cpp

@@ -996,7 +996,8 @@ WebRTCSession::addVideoPipeline(int vp8PayloadType)
         g_signal_emit_by_name(webrtcbin, "get-transceivers", &transceivers);
         GstWebRTCRTPTransceiver *transceiver =
           g_array_index(transceivers, GstWebRTCRTPTransceiver *, 1);
-        transceiver->direction = GST_WEBRTC_RTP_TRANSCEIVER_DIRECTION_SENDONLY;
+        g_object_set(
+          transceiver, "direction", GST_WEBRTC_RTP_TRANSCEIVER_DIRECTION_SENDONLY, nullptr);
         g_array_unref(transceivers);
     }