Add missing `gzip` dependency and update to SPDX

When building a minimal image (with mkosi) I noticed that loadkeys fails without gzip installed. Generally it will be installed through base or other packages, but for more minimal images that might not be the case.

License:

• https://git.kernel.org/pub/scm/linux/kernel/git/legion/kbd.git/tree/LICENSE
• https://git.kernel.org/pub/scm/linux/kernel/git/legion/kbd.git/tree/README.md (or later specified at the bottom)