srtp: get ssrc for Application-Defined RTCP Packet

Application-defined RTCP(204) packets also have a ssrc in their header.
To find a valid stream from the packet, the ssrc should be used.

https://bugzilla.gnome.org/show_bug.cgi?id=791955

ext/srtp/gstsrtp.c

@@ -167,6 +167,10 @@ rtcp_buffer_get_ssrc (GstBuffer * buf, guint32 * ssrc)
           *ssrc = gst_rtcp_packet_fb_get_sender_ssrc (&packet);
           ret = TRUE;
           break;
+        case GST_RTCP_TYPE_APP:
+          *ssrc = gst_rtcp_packet_app_get_ssrc (&packet);
+          ret = TRUE;
+          break;
         default:
           break;
       }