mingw: fix calling convention for strftime()

`strftime` uses `__cdecl` and not `__stdcall`.  This was exposed by
refactoring in commit 26c9fbab (MINGW-w64: Fix string(TIMESTAMP) build
on 32bits, 2021-11-16, v3.22.0~1^2).

This fixes a crash creating a timestamp with clang+mingw-w64 targeting x86.

Fixes: #22939