nat: Add new connections to the end of the list

By adding them to the end the VTY interface will only append
connections and not change the order on each invocation.
diff --git a/openbsc/src/nat/bsc_sccp.c b/openbsc/src/nat/bsc_sccp.c
index 176f06a..3906bc9 100644
--- a/openbsc/src/nat/bsc_sccp.c
+++ b/openbsc/src/nat/bsc_sccp.c
@@ -99,7 +99,7 @@
 		return -1;
 	}
 
-	llist_add(&conn->list_entry, &bsc->nat->sccp_connections);
+	llist_add_tail(&conn->list_entry, &bsc->nat->sccp_connections);
 	counter_inc(bsc->cfg->stats.sccp.conn);
 	counter_inc(bsc->cfg->nat->stats.sccp.conn);