Switch libs deps to ostree-libs
    
    On system not based on ostree, there is a following spurious error
      /usr/lib/systemd/system-generators/ostree-system-generator failed with exit status 1
    
    This can be avoided by removing the deps on ostree package on non-ostree
    systems.
    
    On ostree ones, the ostree package can be assumed to be installed.
    Version will still be enforced by forced the right ostree-libs package