vlist: constify key argument to vlist_add

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

vlist.c

@@ -33,7 +33,7 @@ vlist_delete(struct vlist_tree *tree, struct vlist_node *node)
 }
 
 void
-vlist_add(struct vlist_tree *tree, struct vlist_node *node, void *key)
+vlist_add(struct vlist_tree *tree, struct vlist_node *node, const void *key)
 {
 	struct vlist_node *old_node = NULL;
 	struct avl_node *anode;

vlist.h

@@ -51,7 +51,7 @@ static inline void vlist_update(struct vlist_tree *tree)
 	tree->version++;
 }
 
-void vlist_add(struct vlist_tree *tree, struct vlist_node *node, void *key);
+void vlist_add(struct vlist_tree *tree, struct vlist_node *node, const void *key);
 void vlist_delete(struct vlist_tree *tree, struct vlist_node *node);
 void vlist_flush(struct vlist_tree *tree);
 void vlist_flush_all(struct vlist_tree *tree);