package/win32: use _UCRT to enable URCT in mingw-64

_UCRT sets __MSVCRT_VERSION__=0xE00 but not the other way around. _UCRT is needed to enable certain UCRT API. It's also set by MSVC.

Ref !3163 (merged) since we're going to use the UCRT printf API, but it's not actually enabled yet.