m4: Put wireshark plugin into epan/ directory
    
    Since wirshark-2.5.0 toplevel plugins are no longer loaded. Only
    plugins from epan/, wiretap/ or codecs/ subdirs are. Update the
    plugin dir we generate. This is safe to do even for older
    wiresharks, since they load plugins from there too.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
    Reviewed-by: Ján Tomko <jtomko@redhat.com>