ortp: Guard the jitter stats with a version check

ortp 0.18 has introduced the jitter_stats but we only check for
0.21 an later. It is okay that at some point the jitter stats
will be reported. For previous versions it is 0.
diff --git a/src/trau/osmo_ortp.c b/src/trau/osmo_ortp.c
index 7edcd03..65ec269 100644
--- a/src/trau/osmo_ortp.c
+++ b/src/trau/osmo_ortp.c
@@ -553,7 +553,6 @@
 		uint32_t *recv_lost, uint32_t *last_jitter)
 {
 	const rtp_stats_t *stats;
-	const jitter_stats_t *jitter;
 
 	*sent_packets = *sent_octets = *recv_packets = *recv_octets = 0;
 	*recv_lost = *last_jitter = 0;
@@ -568,7 +567,11 @@
 		*recv_lost = stats->cum_packet_loss;
 	}
 
+#if HAVE_ORTP_021
+	const jitter_stats_t *jitter;
+
 	jitter = rtp_session_get_jitter_stats(rs->sess);
 	if (jitter)
 		*last_jitter = jitter->jitter;
+#endif
 }