fix build warning in VirtualMonitorRfbServer

mark VirtualMonitorRfbServer:newClient() as oveeride

fixes warning:
krfb/main-virtualmonitor.cpp:67:23: warning:
 ‘virtual PendingRfbClient* VirtualMonitorRfbServer::newClient(rfbClientPtr)’
 can be marked override [-Wsuggest-override]
   67 |     PendingRfbClient *newClient(rfbClientPtr client) {
      |                       ^~~~~~~~~