Add SPDX-License-Identifier to all source files

Change-Id: I9f2bc8c5d8a2c0bb0dd6caa4de5e2cb11be210e7
diff --git a/src/input/dahdi.c b/src/input/dahdi.c
index 911f862..47276a7 100644
--- a/src/input/dahdi.c
+++ b/src/input/dahdi.c
@@ -6,6 +6,8 @@
  *
  * All Rights Reserved
  *
+ * SPDX-License-Identifier: GPL-2.0+
+ *
  * 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
  * the Free Software Foundation; either version 2 of the License, or