Fix building without libarchive
Fix building without libarchive

Although libarchive is an optional build option, the build
fails without it.

https://bugzilla.gnome.org/show_bug.cgi?id=764056

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