add required copyright notice header

Author: fredemmott

src/TypeSpec/__Private/stringish_cast.hack

@@ -1,3 +1,11 @@
+/*
+ *  Copyright (c) 2017-present, Facebook, Inc.
+ *  All rights reserved.
+ *
+ *  This source code is licensed under the MIT license found in the
+ *  LICENSE file in the root directory of this source tree.
+ *
+ */
 namespace Facebook\TypeSpec\__Private;
 
 function stringish_cast(\Stringish $stringish, string $caller): string {