The problems comes from the way Windows resolves function names among DLL's. It requires special notations to have them available for other DLL's. To this end in Wireshark code the symbol WS_DLL_PUBLIC was introduced. This is added to functions used in plugin dissectors. So far this has not been the case for conversation_set_port2, so it does not have this symbol added to its function definition, hence cannot be used in plugins.