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_sccp.c b/openbsc/src/nat/bsc_sccp.c
index 40676b3..a744d34 100644
--- a/openbsc/src/nat/bsc_sccp.c
+++ b/openbsc/src/nat/bsc_sccp.c
@@ -140,7 +140,8 @@
 		}
 	}
 
-	LOGP(DNAT, LOGL_ERROR, "Unknown connection.\n");
+	LOGP(DNAT, LOGL_ERROR, "Can not remove connection: 0x%x\n",
+	     sccp_src_ref_to_int(parsed->src_local_ref));
 }
 
 /*