getting-started: Add missing system dependencies for tests

Without this dependency, the pifpaf tests won't execute. They will
fail with a cryptic error.

That cryptic error message is currently fixed by pifpaf's author. It's
pending in a PR and should soon land.

Related P341
Related https://github.com/jd/pifpaf/issues/108
Related https://github.com/jd/pifpaf/pull/119