Fixed compile problem for Leonardo after 0bd6a2d

Author: cmaglie

hardware/arduino/avr/cores/arduino/USBAPI.h

@@ -25,6 +25,8 @@ extern USBDevice_ USBDevice;
 //================================================================================
 //	Serial over CDC (Serial1 is the physical port)
 
+struct ring_buffer;
+
 class Serial_ : public Stream
 {
 private:
@@ -193,4 +195,4 @@ void USB_Flush(uint8_t ep);
 
 #endif
 
-#endif /* if defined(USBCON) */
+#endif /* if defined(USBCON) */