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/zsysnum_darwin_amd64.go b/unix/zsysnum_darwin_amd64.go
index 645426a..42792d7 100644
--- a/unix/zsysnum_darwin_amd64.go
+++ b/unix/zsysnum_darwin_amd64.go
@@ -1,6 +1,8 @@
 // mksysnum_darwin.pl /usr/include/sys/unix.h
 // MACHINE GENERATED BY THE ABOVE COMMAND; DO NOT EDIT
 
+// +build amd64,darwin
+
 package unix
 
 const (