IVGCVSW-7024 Add missing license info for reuse lint

Signed-off-by: Jim Flynn <jim.flynn@arm.com>
Change-Id: I97dee6982e0a7be01c13e9e803c0997547a39ff1
diff --git a/third-party/mapbox/optional.hpp b/third-party/mapbox/optional.hpp
index d84705c..b9e682a 100644
--- a/third-party/mapbox/optional.hpp
+++ b/third-party/mapbox/optional.hpp
@@ -1,3 +1,8 @@
+//
+// Copyright (c) MapBox All rights reserved.
+// SPDX-License-Identifier: BSD-3-Clause
+//
+
 #ifndef MAPBOX_UTIL_OPTIONAL_HPP
 #define MAPBOX_UTIL_OPTIONAL_HPP