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/zerrors_linux_amd64.go b/unix/zerrors_linux_amd64.go
index 52d05b1..b40ccb8 100644
--- a/unix/zerrors_linux_amd64.go
+++ b/unix/zerrors_linux_amd64.go
@@ -1,6 +1,8 @@
 // mkerrors.sh -m64
 // MACHINE GENERATED BY THE COMMAND ABOVE; DO NOT EDIT
 
+// +build amd64,linux
+
 // Created by cgo -godefs - DO NOT EDIT
 // cgo -godefs -- -m64 _const.go