virt-wireshark: Properly substract wireshark prefix
    
    So, when building wireshark plugin, we get the plugindir variable
    from the wireshark.pc as well as prefix. Then we replace the
    prefix in the plugindir with our own prefix where libvirt is
    building to:
    
      plugindir="${prefix}${plugindir#ws_prefix}"
    
    However, as you can see, there's '$' missing in front of the
    ws_prefix variable. This results in the mangled plugindir, for
    instance like this:
    
      plugindir='/usr/usr/lib64/wireshark/plugins'
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>