tools: Always compile {virsh,virt-admin}-completer.c
    
    The functions defined in these sources are referenced all over
    the place, however, compiler only when building with readline.
    Thus when building without it linker gets sad as it can't find
    them.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
    Reviewed-by: Erik Skultety <eskultet@redhat.com>