cmd/callgraph: exclude test for Android

For golang/go#11811.

Change-Id: I4cec398a0a6d43f3297c205f73193522928a5b31
Reviewed-on: https://go-review.googlesource.com/13280
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>

cmd/callgraph/main_test.go

@@ -1,3 +1,11 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// No testdata on Android.
+
+// +build !android
+
 package main
 
 import (