crypto/chacha20poly1305/internal/chacha20: add missing copyright header

The file was originally added in 2016.

Change-Id: I1b5c01400fb73e83f39c086ea1235a948d27308d
Reviewed-on: https://go-review.googlesource.com/35874
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

chacha20poly1305/internal/chacha20/chacha_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 chacha20
 
 import (