chore(cpu-template-helper): Add copyright to mock kernel source file

The C code for x86_64 mock kernel was newly added in commit 8377e7f
("chore(cpu-template-helper): Add mock kernels"), but lacked the
copyright.

Signed-off-by: Takahiro Itazuri <[email protected]>

Author: zulinx86

src/cpu-template-helper/src/utils/mock_kernel/x86_64.c

@@ -1,3 +1,6 @@
+// Copyright 2024 Amazon.com, Inc. or its affiliates. All Rights Reserved.
+// SPDX-License-Identifier: Apache-2.0
+
 void _start() {
         while (1) ;
 }