third_party: add libunwind license to debian/copyright

Part of tarantool#6998

NO_DOC=copyright
NO_TEST=copyright
NO_CHANGELOG=copyright

Author: Gumix

debian/copyright

@@ -361,6 +361,10 @@ Files: third_party/metrics/*
 Copyright: 2018-2023 Tarantool
 License: MIT
 
+Files: third_party/libunwind/*
+Copyright: 2002 Hewlett-Packard Co.
+License: MIT
+
 License: BSD-2-Clause
  Redistribution and use in source and binary forms, with or without
  modification, are permitted provided that the following conditions