wireshark: Fix header of get_message_len()
    
    In wireshark commit ceb8d954 (v1.99.2) they have changed the
    signature of a function that determines how long a libvirt packet
    is. Now it accepts a void pointer for passing data into the
    function. Well, this is nice, but we don't need it right now.
    Anyway, we have to change our code.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>