vsh: Provide cmdComplete stub for readline disabled builds
    
    When building without readline, this function does nothing but
    return false. Without touching any of its arguments which
    triggers a build error. Therefore, provide a stub that has
    arguments marked as unused.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
    Reviewed-by: Erik Skultety <eskultet@redhat.com>