win32: ensure that `localtime_r()` is declared even in i686 builds

win32: ensure that `localtime_r()` is declared even in i686 builds

The `__MINGW64__` constant is defined, surprise, surprise, only when
building for a 64-bit CPU architecture.

Therefore using it as a guard to define `_POSIX_C_SOURCE` (so that
`localtime_r()` is declared, among other functions) is not enough, we
also need to check `__MINGW32__`.

Technically, the latter constant is defined even for 64-bit builds. But
let's make things a bit easier to understand by testing for both
constants.

Making it so fixes this compile warning (turned error in GCC v14.1):

  archive-zip.c: In function 'dos_time':
  archive-zip.c:612:9: error: implicit declaration of function 'localtime_r';
  did you mean 'localtime_s'? [-Wimplicit-function-declaration]
    612 |         localtime_r(&time, &tm);
        |         ^~~~~~~~~~~
        |         localtime_s

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>