Add SPDX annotation

Related: OS#3515
Change-Id: I3719bd8dc015569ecd81928fc079e27593cdca09
diff --git a/GSM/GSMCommon.h b/GSM/GSMCommon.h
index a3ada26..48723b4 100644
--- a/GSM/GSMCommon.h
+++ b/GSM/GSMCommon.h
@@ -2,6 +2,8 @@
 /*
 * Copyright 2008-2011 Free Software Foundation, Inc.
 *
+* SPDX-License-Identifier: AGPL-3.0+
+*
 * This software is distributed under the terms of the GNU Affero Public License.
 * See the COPYING file in the main directory for details.
 *