added end() method

Anticipating the end() method being called by the underlying MIDI library

Author: lathoub

src/USB-MIDI.h

@@ -61,6 +61,10 @@ class usbMidiTransport
         mRxLength = 0;
     };
 
+    void end()
+    {
+    }
+
 	bool beginTransmission(MidiType status)
 	{
         mTxStatus = status;