tool: fix license for build.rs

Most of the code in build.rs is from main.rs and therefore now has the same copyright and license

Signed-off-by: Charlotte Fulham <u7917382@anu.edu.au>

Author: sfulham

tool/microkit/src/build.rs

@@ -1,3 +1,9 @@
+//
+// Copyright 2025, UNSW
+//
+// SPDX-License-Identifier: BSD-2-Clause
+//
+
 use std::{
     collections::HashMap,
     fs::{self, metadata},