crypto/x509: add missing copyright

Change-Id: Ida3b431a06527f6cd604ab4af5ce517959c8619b
Reviewed-on: https://go-review.googlesource.com/2306Reviewed-by: Dave Cheney <dave@cheney.net>

src/crypto/x509/root_darwin_test.go

+// Copyright 2013 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.
+
 package x509
 
 import "testing"