windows: gofmt to add go:build lines

Change-Id: I71b9023fcb6c9860ea35ba0d2cf77a6eed5176b9
Reviewed-on: https://go-review.googlesource.com/c/sys/+/357329
Reviewed-by: Jason A. Donenfeld <Jason@zx2c4.com>
Reviewed-by: David Crawshaw <crawshaw@golang.org>
Trust: Jason A. Donenfeld <Jason@zx2c4.com>
Trust: David Crawshaw <crawshaw@golang.org>
Run-TryBot: Jason A. Donenfeld <Jason@zx2c4.com>
TryBot-Result: Go Bot <gobot@golang.org>
diff --git a/windows/syscall_test.go b/windows/syscall_test.go
index af3ff32..fd4a015 100644
--- a/windows/syscall_test.go
+++ b/windows/syscall_test.go
@@ -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 windows
 // +build windows
 
 package windows_test