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 af9e6331ae)

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

src/include/port/win32ntdll.h

@@ -11,6 +11,9 @@
  *-------------------------------------------------------------------------
  */
 
+#ifndef WIN32NTDLL_H
+#define WIN32NTDLL_H
+
 /*
  * Because this includes NT headers that normally conflict with Win32 headers,
  * any translation unit that includes it should #define UMDF_USING_NTSTATUS
@@ -25,3 +28,5 @@ typedef NTSTATUS (__stdcall *RtlGetLastNtStatus_t) (void);
 extern RtlGetLastNtStatus_t pg_RtlGetLastNtStatus;
 
 extern int	initialize_ntdll(void);
+
+#endif							/* WIN32NTDLL_H */

src/tools/pginclude/cpluspluscheck

@@ -75,6 +75,7 @@ do
 	test "$f" = src/include/port/win32/sys/socket.h && continue
 	test "$f" = src/include/port/win32_msvc/dirent.h && continue
 	test "$f" = src/include/port/win32_msvc/utime.h && continue
+	test "$f" = src/include/port/win32ntdll.h && continue
 	test "$f" = src/port/pthread-win32.h && continue
 
 	# Likewise, these files are platform-specific, and the one

src/tools/pginclude/headerscheck

@@ -71,6 +71,7 @@ do
 	test "$f" = src/include/port/win32/sys/socket.h && continue
 	test "$f" = src/include/port/win32_msvc/dirent.h && continue
 	test "$f" = src/include/port/win32_msvc/utime.h && continue
+	test "$f" = src/include/port/win32ntdll.h && continue
 	test "$f" = src/port/pthread-win32.h && continue
 
 	# Likewise, these files are platform-specific, and the one