Add (void) parameter to methods expected for wireshark plugins, because ...

...the build process expects them via regexps.

This should fix the build with Wireshark 1.10.x, it is tested on 1.10.7.

Please note that using the plugin still doesn't work because 1.10.7 contains a bitcoin dissector (although apparently an old one). This leads wireshark to crash on startup when the plugin is also present.