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>