windows: add //go:build line to empty.s

Change-Id: I715846360d6f889c64b59348e3483ecf8a99f428
Trust: Tobias Klauser <>
Run-TryBot: Tobias Klauser <>
TryBot-Result: Go Bot <>
Reviewed-by: Ian Lance Taylor <>
diff --git a/windows/empty.s b/windows/empty.s
index 69309e4..fdbbbcd 100644
--- a/windows/empty.s
+++ b/windows/empty.s
@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
+//go:build !go1.12
 // +build !go1.12
 // This file is here to allow bodyless functions with go:linkname for Go 1.11