osmo-ortp: Prefix oRTP messages with osmo-ortp

When many applications write to the stdout it is not clear that
'network_error' comes from osmo-bts due the usage of oRTP. At least
provide a hint that oRTP is involved.
diff --git a/src/trau/osmo_ortp.c b/src/trau/osmo_ortp.c
index 64cb305..a9503ea 100644
--- a/src/trau/osmo_ortp.c
+++ b/src/trau/osmo_ortp.c
@@ -102,22 +102,22 @@
 
 static void ortp_sig_cb_ssrc(RtpSession *rs, void *data)
 {
-	fprintf(stderr, "ssrc_changed\n");
+	fprintf(stderr, "osmo-ortp: ssrc_changed\n");
 }
 
 static void ortp_sig_cb_pt(RtpSession *rs, void *data)
 {
-	fprintf(stderr, "payload_type_changed\n");
+	fprintf(stderr, "osmo-ortp: payload_type_changed\n");
 }
 
 static void ortp_sig_cb_net(RtpSession *rs, void *data)
 {
-	fprintf(stderr, "network_error\n");
+	fprintf(stderr, "osmo-ortp: network_error\n");
 }
 
 static void ortp_sig_cb_ts(RtpSession *rs, void *data)
 {
-	fprintf(stderr, "timestamp_jump\n");
+	fprintf(stderr, "osmo-rtp: timestamp_jump\n");
 }