nat: Add NAT_NODE and BSC_NODE for the nat

Implement the go to parent and exit functions for the nat.
diff --git a/openbsc/include/openbsc/vty.h b/openbsc/include/openbsc/vty.h
index ed21e88..3346a2c 100644
--- a/openbsc/include/openbsc/vty.h
+++ b/openbsc/include/openbsc/vty.h
@@ -30,6 +30,8 @@
 	NS_NODE,
 	BSSGP_NODE,
 	OML_NODE,
+	NAT_NODE,
+	BSC_NODE,
 };
 
 #endif