hnbgw: vty: set explicit go_parent_cb

A second level of depth will be added to the hnbgw node soon, which will need
explicit go-parent logic.

Change-Id: I8d1c18a396c215e8425ae49872b5c73316087d7d
diff --git a/include/osmocom/iuh/hnbgw.h b/include/osmocom/iuh/hnbgw.h
index 92d7674..850c4d9 100644
--- a/include/osmocom/iuh/hnbgw.h
+++ b/include/osmocom/iuh/hnbgw.h
@@ -148,3 +148,4 @@
 void hnb_context_release(struct hnb_context *ctx);
 
 void hnbgw_vty_init(struct hnb_gw *gw, void *tall_ctx);
+int hnbgw_vty_go_parent(struct vty *vty);