Add missing copyright header (#382)

src/key_exchange/sigma_i/message.rs

@@ -1,3 +1,11 @@
+// Copyright (c) Meta Platforms, Inc. and affiliates.
+//
+// This source code is dual-licensed under either the MIT license found in the
+// LICENSE-MIT file in the root directory of this source tree or the Apache
+// License, Version 2.0 found in the LICENSE-APACHE file in the root directory
+// of this source tree. You may select, at your option, one of the above-listed
+// licenses.
+
 use core::ops::Add;
 
 use derive_where::derive_where;