do not publish plugin functions

[alistairking]

currently the CORSARO_PLUGIN_GENERATE_PROTOS macro creates prototypes for all functions in the plugin API, thus exposing them for use by any corsaro internal code. This is not needed if the appropriate plugin_alloc function passes pointers to the functions to the plugin manager.

This might be as simple as removing the call to CORSARO_PLUGIN_GENERATE_PROTOS from each plugin. Maybe.