dracut: exclude clevis due to missing systemd module

If clevis is available, dracut will add it to the image.
However, as dracut-systemd is already omitted, clevis will
crash complaining of a missing environment variable.

Author: zip

etc/zfsbootmenu/dracut.conf.d/zfsbootmenu.conf

@@ -1,3 +1,3 @@
 nofsck="yes"
 add_dracutmodules+=" zfsbootmenu bash "
-omit_dracutmodules+=" btrfs zfs resume systemd systemd-initrd dracut-systemd plymouth dash "
+omit_dracutmodules+=" btrfs zfs resume systemd systemd-initrd dracut-systemd plymouth dash clevis "