windows: don't return EINVAL on zero Chmod mode

To match CL 174320

Updates golang/go#20858

Change-Id: Ic4e332c8a84b7d427fc7057aa5b0c2877cc9f7ea
Reviewed-on: https://go-review.googlesource.com/c/sys/+/174321
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

windows/syscall_windows.go

@@ -561,9 +561,6 @@ func Fsync(fd Handle) (err error) {
 }
 
 func Chmod(path string, mode uint32) (err error) {
-	if mode == 0 {
-		return syscall.EINVAL
-	}
 	p, e := UTF16PtrFromString(path)
 	if e != nil {
 		return e