Silence warnings for free()s

src/memory.c

@@ -57,11 +57,11 @@ void nl_object_delete(struct nl_object *obj) {
             obj->cell.cdr = NULL;
             break;
         case NL_TYPE_SYMBOL:
-            free(obj->symbol);
+            free((void *) obj->symbol);
             obj->symbol = NULL;
             break;
         case NL_TYPE_STRING:
-            free(obj->string);
+            free((void *) obj->string);
             obj->string = NULL;
         case NL_TYPE_INT:
         case NL_TYPE_COUNT: