internal/syscall/windows: fix handle leak in Mkdirat

Mkdirat does not close the handle returned by CreateFile, but it should.

Mkdirat has been introduced in this developer cycle, so it is not
necessary to backport this fix to any release branch.

Change-Id: Icddac5ccdc6a142a5be5392a39aba2ae7cc9c69a
Reviewed-on: https://go-review.googlesource.com/c/go/+/620195
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Quim Muntal <quimmuntal@gmail.com>

src/internal/syscall/windows/at_windows.go

@@ -164,5 +164,6 @@ func Mkdirat(dirfd syscall.Handle, name string, mode uint32) error {
 	if err != nil {
 		return ntCreateFileError(err, 0)
 	}
+	syscall.CloseHandle(h)
 	return nil
 }