misc: Fix the return type of the go back vty function...

Without this we will get warnings about incompatible assignments
diff --git a/openbsc/src/common_vty.c b/openbsc/src/common_vty.c
index 440209a..16d6815 100644
--- a/openbsc/src/common_vty.c
+++ b/openbsc/src/common_vty.c
@@ -35,7 +35,7 @@
 #include <osmocom/vty/vty.h>
 
 
-int bsc_vty_go_parent(struct vty *vty)
+enum node_type bsc_vty_go_parent(struct vty *vty)
 {
 	switch (vty->node) {
 	case GSMNET_NODE: