[libc++] Add missing xlocale.h include on Apple and FreeBSD (#99689)

Summary:
The `<locale>` header uses `strtoll_l` and friends which are defined in
`<xlocale.h>` on these platforms. While this works via transitive
includes when modules are disabled, this doesn't work anymore if the
platforms are modularized properly.

Test Plan: 

Reviewers: 

Subscribers: 

Tasks: 

Tags: 


Differential Revision: https://phabricator.intern.facebook.com/D60250520

Author: ldionne

libcxx/include/locale

@@ -232,6 +232,10 @@ template <class charT> class messages_byname;
 #    include <__locale_dir/locale_base_api/bsd_locale_fallbacks.h>
 #  endif
 
+#  if defined(__APPLE__) || defined(__FreeBSD__)
+#    include <xlocale.h>
+#  endif
+
 #  if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #    pragma GCC system_header
 #  endif