util: Add a copyright to gem5img.py.

util: Add a copyright to gem5img.py.

This script was ported to python from a bash script by me back in 2011.
The original file didn't have a copyright, but since I made significant
modifications to it (porting it to python, improving its features), at
least those modifications should have become copyright Google.

Change-Id: Ia70bb1e6be5b188537bcf6899ba5884b359dbe18
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/35875
Reviewed-by: Richard Cooper <richard.cooper@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>