TBR: swift/discovery: add build tag

Change-Id: I2d8c36875b932d84bc048f2af3223564c268a12d
diff --git a/v23/discovery/swift.go b/v23/discovery/swift.go
index 0a6f65a..92c1ad5 100644
--- a/v23/discovery/swift.go
+++ b/v23/discovery/swift.go
@@ -1,4 +1,4 @@
-// Copyright 2015 The Vanadium Authors. All rights reserved.
+// Copyright 2016 The Vanadium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
@@ -86,11 +86,11 @@
 	go func() {
 		for update := range ch {
 			data := struct {
-				IsLost        bool
-				Ad            discovery.Advertisement
+				IsLost bool
+				Ad     discovery.Advertisement
 			}{
-				IsLost:        update.IsLost(),
-				Ad:            update.Advertisement(),
+				IsLost: update.IsLost(),
+				Ad:     update.Advertisement(),
 			}
 			b, err := json.Marshal(data)
 			if err != nil {
diff --git a/v23/discovery/util.go b/v23/discovery/util.go
index ce7470e..3b24c89 100644
--- a/v23/discovery/util.go
+++ b/v23/discovery/util.go
@@ -1,3 +1,9 @@
+// Copyright 2016 The Vanadium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build darwin
+
 package discovery
 
 import (
@@ -54,4 +60,4 @@
 		ret = append(ret, cstr.toString())
 	}
 	return ret
-}
\ No newline at end of file
+}