license: add spdx identifier to all files

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Ifddd07cc0eb5edeb2dcb410747073d68c6631cb1
diff --git a/src/responder.c b/src/responder.c
index 5112f7b..9940fa0 100644
--- a/src/responder.c
+++ b/src/responder.c
@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: Apache-2.0 OR GPL-2.0-or-later */
 #include "responder.h"
 
 #include <libpldm/base.h>