Added SPDX identifier.

crx-extract

@@ -1,6 +1,6 @@
 #!/usr/bin/env python3.6
 #
-# Copyright (C) 2017 The University of Sheffield, UK
+# Copyright (C) 2017-2018 The University of Sheffield, UK
 #
 # This program is free software: you can redistribute it and/or modify
 # it under the terms of the GNU General Public License as published by
@@ -15,6 +15,7 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <https://www.gnu.org/licenses/>.
 #
+# SPDX-License-Identifier: GPL-3.0-or-later
 """Tool for extracting crx file from a tar archive."""
 
 import os