vulndb/internal/audit: add missing copyright header

Change-Id: I3d1b59304b18d730dee148142cf1286a26f9da18
Reviewed-on: https://go-review.googlesource.com/c/exp/+/365594
Trust: Hyang-Ah Hana Kim <hyangah@gmail.com>
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Zvonimir Pavlinovic <zpavlinovic@google.com>

vulndb/internal/audit/order_test.go

@@ -1,3 +1,7 @@
+// Copyright 2021 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 audit
 
 import (