cmd/link: add copyright header

Change-Id: I44f57019bb8e659d4aa3da8b13e8bd9a20b9d2e1
Reviewed-on: https://go-review.googlesource.com/c/go/+/253920
Reviewed-by: Than McIntosh <[email protected]>

Author: cherrymui

src/cmd/link/link_test.go

@@ -1,3 +1,7 @@
+// Copyright 2016 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 main
 
 import (