Add virtual beginMulticast(...) stub to UDP class

cores/arduino/Udp.h

@@ -42,6 +42,7 @@ class UDP : public Stream {
 
 public:
   virtual uint8_t begin(uint16_t) =0;  // initialize, start listening on specified port. Returns 1 if successful, 0 if there are no sockets available to use
+  virtual uint8_t beginMulticast(IPAddress, uint16_t) { return 0; }  // initialize, start listening on specified multicast IP address and port. Returns 1 if successful, 0 on failure
   virtual void stop() =0;  // Finish with the UDP socket
 
   // Sending UDP packets