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/alloc.c b/alloc.c
index 1e7da5a..9edfd6c 100644
--- a/alloc.c
+++ b/alloc.c
@@ -1,4 +1,4 @@
-/* SPDX-License-Identifier: Apache-2.0 */
+/* SPDX-License-Identifier: Apache-2.0 OR GPL-2.0-or-later */
 
 #include <assert.h>