scrypt: add missing license header

Change-Id: I17daa73c1957ed276ee32419248f321e018f5091
Reviewed-on: https://go-review.googlesource.com/67316
Reviewed-by: Kevin Burke <kev@inburke.com>

scrypt/example_test.go

@@ -1,3 +1,7 @@
+// Copyright 2017 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 scrypt_test
 
 import (