license: add spdx identifier to all files

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Ifddd07cc0eb5edeb2dcb410747073d68c6631cb1
diff --git a/src/base.c b/src/base.c
index 908621b..4322ef2 100644
--- a/src/base.c
+++ b/src/base.c
@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: Apache-2.0 OR GPL-2.0-or-later */
 #include "base.h"
 #include "pldm_types.h"
 #include <endian.h>