use int main(void) for main, so clang won't complain about unused argc/argv

linked-list.c

@@ -122,7 +122,7 @@ list_node_t* list_elementat(list_t *list, int i) {
   return cur;
 }
 
-int main(int argc, char *argv[]) {
+int main(void) {
 
   /* Test empty list */
   list_t *emptylist = malloc(sizeof(list_t));