build: add REUSE Compliance Check

In accordance with reuse requirements:
- Place each license file in the LICENSES/ directory
- Add missing SPDX-License-Identifier to files.
- Add .reuse/dep5 to bulk-license files

Fixes: #5887

Signed-off-by: Ruslan Mstoi <ruslan.mstoi@intel.com>

virtio-devices/src/mem.rs

@@ -1,5 +1,7 @@
 // Copyright (c) 2020 Ant Financial
 //
+// SPDX-License-Identifier: Apache-2.0
+//
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
 // You may obtain a copy of the License at