gbproxy_main: Add ctrl vty commands

osmo-gbproxy already provides a ctrl interface, we should be able to
change ctrl-related configuration.

Change-Id: I3f3aa46aa032c1bd0ec88163bb89701d2250f414

src/gb_proxy_main.c

@@ -280,6 +280,7 @@ int main(int argc, char **argv)
 
 	vty_info.copyright = openbsc_copyright;
 	vty_init(&vty_info);
+	ctrl_vty_init(tall_sgsn_ctx);
 	logging_vty_add_cmds();
 	osmo_talloc_vty_add_cmds();
 	osmo_stats_vty_add_cmds();