[build] add missing newlines at end of files

As discussed here: https://gitlab.com/nomadic-labs/albert/-/merge_requests/new?merge_request%5Bsource_branch%5D=rafoo%40newlines many files in the repo fail to respect the final newline convention. This commit fixes them; it was produced by the following zsh command:

for f in **/*; do tail -c 1 $f | od -ta | grep -q nl
if [ $? -eq 1 ]
then
echo >> $f
fi
done

taken from https://it.toolbox.com/question/how-to-add-end-of-line-character-to-files-that-do-not-have-the-end-of-line-062708.

This script has been adapted for enforcing this in the CI.