[libc++][NFC] Add missing include to ext/__hash

[ldionne]

Summary

Note that we use <stddef.h> instead of <cstddef> because we're
using size_t (not std::size_t) and we're outside of namespace std.

[jloser]

LGTM!

libcxx/include/ext/__hash

 // -*- C++ -*-
 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef _LIBCPP_EXT_HASH
 #define _LIBCPP_EXT_HASH
 
 #  pragma GCC system_header
 
 #include <__config>
 #include <cstring>
+#include <stddef.h>
 #include <string>
 
 namespace __gnu_cxx {
 
 template <typename _Tp> struct _LIBCPP_TEMPLATE_VIS hash { };
 
 template <> struct _LIBCPP_TEMPLATE_VIS hash<const char*>
  : public std::__unary_function<const char*, size_t>