[PATCH] Fix build failure on macOS 26.2 SDK due to missing nl_langinfo_l declaration

Hi,

Building PostgreSQL against the macOS 26.2 SDK (Xcode 26 beta) fails
with the following error:

src/port/chklocale.c:326:8: error: call to undeclared function
'nl_langinfo_l'; ISO C99 and later do not support implicit function
declarations [-Wimplicit-function-declaration]
sys = nl_langinfo_l(CODESET, loc);

== Root Cause ==

macOS 26.2 SDK changed the structure of <langinfo.h>. In previous
SDK versions, nl_langinfo_l() was declared unconditionally. In the
new SDK, it is only exposed when _USE_EXTENDED_LOCALES_ is defined,
guarded behind a conditional include of <xlocale/_langinfo.h>:

/* macOS 26.2 SDK: langinfo.h */
#include <_langinfo.h> /* nl_langinfo() only */

#ifdef _USE_EXTENDED_LOCALES_
#include <xlocale/_langinfo.h> /* nl_langinfo_l() */
#endif

PostgreSQL's chklocale.c only includes <langinfo.h> directly, so
nl_langinfo_l() is no longer visible on macOS 26.2.

== Fix ==

Include <xlocale.h> explicitly on Apple platforms. This header
declares nl_langinfo_l() unconditionally and is the standard way
to access locale-specific extensions on macOS.

Tested on macOS 26.2 (darwin 25.3) with Apple Clang 17.

Patch attached.

Regards,
Jianghua Yang