Include `systemd.volatile` infrastructure in initrd
Include `systemd.volatile` infrastructure in initrd

For the `systemd.volatile=...` feature to work, the
`systemd-volatile-root` generator and `systemd-volatile-root.service`
service are required to be present. Booting a read-only image generated
by `mkosi` (e.g., `gpt_ext4` built with the `--read-only` flag, or a
`gpt_squashfs` image) with a volatile but mutable overlay is useful, so
we need to provide these files in the `initrd` Dracut generates.

Hence, adding both files to `DRACUT_SYSTEMD_EXTRAS`.

Fixes: #674
See: https://github.com/systemd/mkosi/issues/674

systemd/mkosi,systemd/mkosi