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/bsc_msc.c b/openbsc/src/bsc_msc.c
index f718b55..22523bf 100644
--- a/openbsc/src/bsc_msc.c
+++ b/openbsc/src/bsc_msc.c
@@ -84,6 +84,7 @@
 	fd->when = BSC_FD_READ;
 
 	con->is_connected = 1;
+	LOGP(DMSC, LOGL_NOTICE, "(Re)Connected to the MSC.\n");
 	if (con->connected)
 		con->connected(con);
 	return 0;