Add SPDX-License-Identifier to all source files

Change-Id: I9f2bc8c5d8a2c0bb0dd6caa4de5e2cb11be210e7
diff --git a/src/trau/osmo_ortp.c b/src/trau/osmo_ortp.c
index c49a23d..fab020d 100644
--- a/src/trau/osmo_ortp.c
+++ b/src/trau/osmo_ortp.c
@@ -2,6 +2,8 @@
  * (C) 2011 by On-Waves e.h.f
  * 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