pgsql: Add missing include guard to win32ntdll.h.

Add missing include guard to win32ntdll.h.

Oversight in commit e2f0f8ed.  Also add this file to the exclusion lists
in headerscheck and cpluscpluscheck, because Unix systems don't have a
header it includes.

Reported-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/2760528.1641929756%40sss.pgh.pa.us
(cherry picked from commit af9e6331aeba149c93052c3549140082a85a3cf9)

Author: Thomas Munro <tmunro@postgresql.org>
Author: Alexandra Wang <alexandra.wang.oss@gmail.com>

Branch
------
REL_13_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/5c0b7581ba12bb01ebebce60583e3db6c0511057
Author: Thomas Munro <tmunro@postgresql.org>

Modified Files
--------------
src/include/port/win32ntdll.h      | 5 +++++
src/tools/pginclude/cpluspluscheck | 1 +
src/tools/pginclude/headerscheck   | 1 +
3 files changed, 7 insertions(+)