chore: add reuse licence checking (#6659)

This PR aims to add very basic REUSE license checking
https://reuse.software.

My intention is not to sort out all errors in this PR, but instead sort
that out over multiple incrementals PR going forward in an undisruptive
way as possible.

lib/lrucache/lrucache/lrucache.hpp

@@ -3,6 +3,9 @@
  * Original Author: Alexander Ponomarev
  *
  * Created on June 20, 2013, 5:09 PM
+ *
+ * SPDX-FileCopyrightText: Copyright (c) 2014, lamerman
+ * SPDX-License-Identifier: BSD-3-Clause
  */
 
 #ifndef _LRUCACHE_HPP_INCLUDED_