LICENSE: add GPLv2 license option.

As per the MCTP design document
(https://github.com/openbmc/docs/blob/master/designs/mctp.md), a
dual-licence (GPLv2+ and Apache-2.0) gives us flexibility with including
libcmtp code into various firmware environments.

This change adds GPLv2+ as a license option. All current contributors
have agreed to this change.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
diff --git a/tests/test-utils.h b/tests/test-utils.h
index b565876..94f43b8 100644
--- a/tests/test-utils.h
+++ b/tests/test-utils.h
@@ -1,3 +1,5 @@
+/* SPDX-License-Identifier: Apache-2.0 OR GPL-2.0-or-later */
+
 #ifndef _MCTP_TESTS_TEST_UTILS_H
 #define _MCTP_TESTS_TEST_UTILS_H