use signed int to make pedantic gcc happy

linked-list.c

@@ -109,7 +109,7 @@ void list_revert(list_t *list) {
   list->head = reverted.head;
 }
 
-list_node_t* list_elementat(list_t *list, unsigned int i) {
+list_node_t* list_elementat(list_t *list, int i) {
   if (list_empty(list) || i > list_length(list)-1) {
     return NULL;
   }