Add BuildRequires: systemd
    
    Quoting http://fedoraproject.org/wiki/Packaging:Systemd
    
      Please note that in order for the %{_unitdir} macro to exist, your
      package must have:
    
      BuildRequires: systemd-units
    
      On Fedora 18 and newer, the systemd-units subpackage has been merged
      into the base systemd package, so your package for those targets can
      instead include:
    
      BuildRequires: systemd
    
    See, "must have".