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>

mingw-w64-headers/crt/corecrt.h

diff --git a/mingw-w64-headers/crt/corecrt.h b/mingw-w64-headers/crt/corecrt.h
index c3f3a82..e0f0601 100644
--- a/mingw-w64-headers/crt/corecrt.h
+++ b/mingw-w64-headers/crt/corecrt.h
@@ -141,6 +141,7 @@
 #endif
 #endif /* _TIME_T_DEFINED */
 
+#ifndef __CYGWIN__ /* Cygwin defines mbstate_t themselves */
 #if defined(_UCRT) || defined(__LARGE_MBSTATE_T)
   typedef struct _Mbstatet {
     unsigned long _Wchar;
@@ -150,6 +151,7 @@
 #else
   typedef int mbstate_t;
 #endif
+#endif
 
 #ifndef _CRT_SECURE_CPP_NOTHROW
 #define _CRT_SECURE_CPP_NOTHROW throw()