libc: correct SPDX tag on strstr.c

It was obtained from musl, and is MIT licensed.

MFC after:	3 days
Sponsored by:	The FreeBSD Foundation

lib/libc/string/strstr.c

 /*-
- * SPDX-License-Identifier: BSD-3-Clause
+ * SPDX-License-Identifier: MIT
  *
  * Copyright (c) 2005-2014 Rich Felker, et al.
  *