net/http: append servermux copyright headers

Updates golang#63682

Author: eudore

src/net/http/servemux.go

@@ -1,3 +1,7 @@
+// Copyright 2009 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 http
 
 import (

src/net/http/servemux_test.go

@@ -1,3 +1,7 @@
+// Copyright 2009 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 http_test
 
 import (