Add an empty flushTX to MarlinSerialUSB_Due

Marlin/src/HAL/HAL_DUE/MarlinSerialUSB_Due.cpp

       (usb_task_cdc_isenabled() && udi_cdc_is_rx_ready());
 }
 
-void MarlinSerialUSB::flush(void) {
-}
+void MarlinSerialUSB::flush(void) { }
+void MarlinSerialUSB::flushTX(void) { }
 
 void MarlinSerialUSB::write(const uint8_t c) {
 

Marlin/src/HAL/HAL_DUE/MarlinSerialUSB_Due.h

   static int peek(void);
   static int read(void);
   static void flush(void);
+  static void flushTX(void);
   static bool available(void);
   static void write(const uint8_t c);