internal/reflectlite: add missing copyright header

Change-Id: Id43870de6608ef2e8c0ebef82fd710b2c3061e66
Reviewed-on: https://go-review.googlesource.com/c/go/+/199599Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

src/internal/reflectlite/reflect_mirror_test.go

+// Copyright 2019 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 reflectlite_test
 
 import (