audio: skip build for non-darwin.

Change-Id: If382b517a21b90412ced3bfa543575055df8de3a
Reviewed-on: https://go-review.googlesource.com/5510
Reviewed-by: Burcu Dogan <jbd@google.com>

audio/audio.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 darwin
+
 // Package audio provides a basic audio player.
 package audio
 

audio/audio_test.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 darwin
+
 package audio
 
 import (