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>