runtime: make errno positive in netpollopen

Make netpollopen return what Windows GetLastError API returns.
It is probably copy / paste error from long time ago.

Change-Id: I28f78718c15fef3e8b5f5d11a259533d7e9c6185
Reviewed-on: https://go-review.googlesource.com/72592Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>

src/runtime/netpoll_windows.go

@@ -47,7 +47,7 @@ func netpolldescriptor() uintptr {
 
 func netpollopen(fd uintptr, pd *pollDesc) int32 {
 	if stdcall4(_CreateIoCompletionPort, fd, iocphandle, 0, 0) == 0 {
-		return -int32(getlasterror())
+		return int32(getlasterror())
 	}
 	return 0
 }