[libc++][Modules] locale fails to compile with clang modules when _LIBCPP_LOCALE__L_EXTENSIONS is undefined

When `__locale_dir/locale_base_api/locale_guard.h is` compiled independently, as it is when it's in its own clang module, it fails to compile due to `locale_t` being undefined. It needs to include `__locale` to get that, instead of just `clocale`.

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D158669

Author: ian-twilightcoder

libcxx/include/__locale_dir/locale_base_api/locale_guard.h

@@ -10,6 +10,7 @@
 #define _LIBCPP___LOCALE_LOCALE_BASE_API_LOCALE_GUARD_H
 
 #include <__config>
+#include <__locale> // for locale_t
 #include <clocale>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)