MinGW cross compilation failure on Debian trixie

Hi,

I am working on upgrading Debian bookworm CI images to Debian trixie.
CompilerWarnings CI task's mingw_cross_warning_script failed with [1]:

```
make -s -j${BUILD_JOBS} clean
time make -s -j${BUILD_JOBS} world-bin
In file included from ../../../../src/include/port.h:16,
from ../../../../src/include/c.h:1353,
from ../../../../src/include/postgres.h:48,
from pg_locale_libc.c:12:
pg_locale_libc.c: In function ‘wc_isdigit_libc_mb’:
../../../../src/include/port/win32_port.h:438:20: error: implicit
declaration of function ‘_iswctype_l’; did you mean ‘_isctype_l’?
[-Wimplicit-function-declaration]
438 | #define iswdigit_l _iswdigit_l
| ^~~~~~~~~~~
pg_locale_libc.c:178:16: note: in expansion of macro ‘iswdigit_l’
178 | return iswdigit_l((wint_t) wc, locale->info.lt);
| ^~~~~~~~~~
make[4]: *** [<builtin>: pg_locale_libc.o] Error 1
make[4]: *** Waiting for unfinished jobs....
make[3]: *** [../../../src/backend/common.mk:37: adt-recursive] Error 2
make[2]: *** [common.mk:37: utils-recursive] Error 2
make[1]: *** [Makefile:42: all-backend-recurse] Error 2
make: *** [GNUmakefile:21: world-bin-src-recurse] Error 2
```

I am not sure if this problem is about Postgres or Debian yet; but I
wanted to share it.

I can reproduce this problem on my Debian sid machine on upstream,
REL_18_STABLE and REL_17_STABLE branches. REL_16_STABLE compiles
successfully. My specs are:
- Linux nbyavuz 6.16.7+deb14-amd64 #1 SMP PREEMPT_DYNAMIC Debian
6.16.7-1 (2025-09-11) x86_64 GNU/Linux
- gcc version 15.2.0 (Debian 15.2.0-4)
- mingw-w64-tools 12.0.0-5
- gcc-mingw-w64-x86-64-win32 14.2.0-19+27+b1
- g++-mingw-w64-x86-64-win32 14.2.0-19+27+b1

[1] https://cirrus-ci.com/task/6240816059908096

--
Regards,
Nazir Bilal Yavuz
Microsoft