switchroot/generator: Add var.mount to local-fs.target.requires
switchroot/generator: Add var.mount to local-fs.target.requires

Unbreaks mounting in CentOS. Newer systemd in Fedora pulls didn't need this, I
think due to `RequiresMountsFor=`. Anyways, this is what the fstab generator
does, and it's clearly right ✓.

Closes: https://github.com/ostreedev/ostree/issues/867

Closes: #869
Approved by: jlebon

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree