CI: add uncrustify config.
One of the major annoying back and forths in reviews, is code formatting issues. To solve this, we can use uncrustify, such as already in use by e.g. nautilus, on which this is also based.
We don't have to do this on all our files, but we can for example make sure that new contributions are first processed through this formatter, or to even add it to the CI (once it's setup).
You can run uncrustify in the repo's root directory with for example
$ uncrustify -c data/uncrustify.cfg --no-backup src/**/*.c