VTY: cosmetic: make struct cmd_node for GSMNET_NODE static

We don't need to expose it anyway.

Change-Id: Icbaed6b3b15aa719a19bb4be8fed347073025a22
diff --git a/src/libmsc/msc_vty.c b/src/libmsc/msc_vty.c
index e2e892a..a629b5c 100644
--- a/src/libmsc/msc_vty.c
+++ b/src/libmsc/msc_vty.c
@@ -69,7 +69,7 @@
 
 static struct gsm_network *gsmnet = NULL;
 
-struct cmd_node net_node = {
+static struct cmd_node net_node = {
 	GSMNET_NODE,
 	"%s(config-net)# ",
 	1,