headers: add missing _W_* lconv struct members

This seems to have been added with MSVCR100 but is also available in MSVCRT
on Windows 7 SP1+ (at least), more info here:
https://sourceforge.net/p/mingw-w64/mailman/message/37183101/

This change is motivated by CPython starting to use these members in v3.8.7

Reference: https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/localeconv
Reference: https://sourceforge.net/p/mingw-w64/bugs/870/
Signed-off-by: Liu Hao <lh_mouse@126.com>