http2: add missing copyright header to export_test.go

Change-Id: I7add2f2ef54ef6f985154a60399768e76a6a6964
Reviewed-on: https://go-review.googlesource.com/c/net/+/751620
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Nicholas Husin <nsh@golang.org>
Reviewed-by: Nicholas Husin <husin@google.com>
Auto-Submit: Damien Neil <dneil@google.com>

Author: neild

http2/export_test.go

@@ -1,3 +1,7 @@
+// Copyright 2026 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 http2
 
 import (