SDL_opengles2_khrplatform.h: add cygwin bit missed in commit 2cd5cd2a.

include/SDL3/SDL_opengles2_khrplatform.h

@@ -122,6 +122,8 @@
 #if defined(_WIN32) && !defined(_WIN32_WCE) && !defined(__SCITECH_SNAP__)
     /* Win32 but not WinCE */
 #   define KHRONOS_APIENTRY __stdcall
+#elif defined(__CYGWIN__) /* __CYGWIN__ added by SDL */
+#   define KHRONOS_APIENTRY __stdcall
 #else
 #   define KHRONOS_APIENTRY
 #endif