headers: corecrt.h: Don't define mbstate_t for Cygwin.

This fixes a regression that was caused by 8c4baed92d4dee4fe1874af860a61aa5c945b6f5.
Cygwin has its own `mbstate_t` which is different from the Windows one.

Signed-off-by: Zhongteng Gui <dragon-archer@outlook.com>
Reference: https://sourceforge.net/p/mingw-w64/mailman/message/59391723/
Signed-off-by: LIU Hao <lh_mouse@126.com>
1 file changed