Define WSAPOLLFD only on mingw 4.x

Fixes #10327

Author: klutzy

src/etc/mingw-fix-include/winsock2.h

@@ -3,10 +3,15 @@
 
 #include_next <winsock2.h>
 
+// mingw 4.0.x has broken headers (#9246) but mingw-w64 does not.
+#if defined(__MINGW_MAJOR_VERSION) && __MINGW_MAJOR_VERSION == 4
+
 typedef struct pollfd {
   SOCKET fd;
   short  events;
   short  revents;
 } WSAPOLLFD, *PWSAPOLLFD, *LPWSAPOLLFD;
 
 #endif
+
+#endif // _FIX_WINSOCK2_H