build: fix for 32-bit windows builds on 64-bit windows system

Thanks to jon.forums@ for the fix.

Fixes #5051.

R=golang-dev, minux.ma
CC=golang-dev
https://golang.org/cl/7813045

include/libc.h

@@ -308,7 +308,7 @@ extern	void	flagprint(int);
 
 #ifdef _WIN32
 
-#ifndef _WIN64
+#if !defined(_WIN64) && !defined(__MINGW64_VERSION_MAJOR)
 struct timespec {
 	int tv_sec;
 	long tv_nsec;