This fixes an issue where pbsplash was not killed when the override is attached to a service that runs as a non-root user (e.g. tinydm.service). The tiny `+` tells systemd to run this command with full privileges, the other stuff in the service's ExecStart and so on are unaffected by this change. This also adds the kill pbsplash override to getty@.service, fixes #4018 Part-of: <https://gitlab.postmarketos.org/postmarketOS/pmaports/-/merge_requests/7202>