unix: add explicit build tags

Change-Id: I62774b8ee0c1a7cc1a3b7009ca860e3fd64a6564
Signed-off-by: Shenghou Ma <minux@golang.org>
Reviewed-on: https://go-review.googlesource.com/10182
Reviewed-by: Rob Pike <r@golang.org>
diff --git a/unix/syscall_linux_amd64.go b/unix/syscall_linux_amd64.go
index dbf57d2..ae70c2a 100644
--- a/unix/syscall_linux_amd64.go
+++ b/unix/syscall_linux_amd64.go
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build amd64,linux
+
 package unix
 
 import "syscall"