x/net/html/charset: add missing copyright

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/174240043

html/charset/charset.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 charset provides common text encodings for HTML documents.
 //
 // The mapping from encoding labels to encodings is defined at

html/charset/gen.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.
+
 // +build ignore
 
 package main