Regression: FTBFS upnp: build broken with upnp 1.8

Since 0d89fe3f vlc no longer builds with upnp 1.8.4:

  CXX      services_discovery/libupnp_plugin_la-upnp.lo                                                                                                       
../../modules/services_discovery/upnp.cpp: In member function ‘virtual int RD::MediaRendererList::onEvent(Upnp_EventType, UpnpEventPtr, void*)’:               ../../modules/services_discovery/upnp.cpp:1525:74: error: cannot convert ‘RD::MediaRendererList::onEvent(Upnp_EventType, UpnpEventPtr, void*)::Upnp_Discovery*$ to ‘const UpnpDiscovery*’ {aka ‘const s_UpnpDiscovery*’}                                                                                                     
             i_res = UpnpDownloadXmlDoc( UpnpDiscovery_get_Location_cstr( p_discovery ), &p_doc);
                                                                          ^~~~~~~~~~~ 
../../modules/services_discovery/upnp.cpp:1531:70: error: cannot convert ‘RD::MediaRendererList::onEvent(Upnp_EventType, UpnpEventPtr, void*)::Upnp_Discovery*’ to ‘const UpnpDiscovery*’ {aka ‘const s_UpnpDiscovery*’}
             parseNewRenderer(p_doc, UpnpDiscovery_get_Location_cstr( p_discovery ) );
                                                                      ^~~~~~~~~~~
../../modules/services_discovery/upnp.cpp:1540:40: error: invalid use of incomplete type ‘struct RD::MediaRendererList::onEvent(Upnp_EventType, UpnpEventPtr, void*)::Upnp_Discovery’
             removeRenderer( p_discovery->DeviceId );
                                        ^~

(warnings and notes removed)