Add license header to idtr.rs and fix imports

Co-authored-by: simongdavies <[email protected]>

Author: Copilot

src/hyperlight_guest/src/exceptions/idtr.rs

@@ -1,3 +1,20 @@
+/*
+Copyright 2024 The Hyperlight Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+use core::mem::size_of;
 use core::ptr::addr_of;
 
 use crate::exceptions::idt::{init_idt, IdtEntry, IDT};