get rid of non-ANSI function declarations missing (void)

Detected by Smatch
diff --git a/src/vty/command.c b/src/vty/command.c
index 5dc1dd4..1c8df61 100644
--- a/src/vty/command.c
+++ b/src/vty/command.c
@@ -152,7 +152,7 @@
 }
 
 /* Sort each node's command element according to command string. */
-void sort_node()
+void sort_node(void)
 {
 	unsigned int i, j;
 	struct cmd_node *cnode;