nat: Logging improvement for the nat.

Print some more information, e.g. when a BSC is connected
and authenticated, print the SCCP ref number.
diff --git a/openbsc/src/nat/bsc_mgcp_utils.c b/openbsc/src/nat/bsc_mgcp_utils.c
index 82eeefc..0c666c0 100644
--- a/openbsc/src/nat/bsc_mgcp_utils.c
+++ b/openbsc/src/nat/bsc_mgcp_utils.c
@@ -213,7 +213,8 @@
 	}
 
 	if (!bsc_endp) {
-		LOGP(DMGCP, LOGL_ERROR, "Could not find active endpoint: %s\n", transaction_id);
+		LOGP(DMGCP, LOGL_ERROR, "Could not find active endpoint: %s for msg: '%s'\n",
+		     transaction_id, (const char *) msg->l2h);
 		return;
 	}