Replace missing USB serialEvent declaration

This was inadvertently removed upstream, although it's needed for the dummy Serial interface used by both XInput and USB_DISABLED.

teensy/avr/cores/teensy4/usb_serial.h

@@ -186,6 +186,7 @@ class usb_serial_class : public Stream
 };
 // Serial provides USB Virtual Serial communication with your computer.
 extern usb_serial_class Serial;
+extern void serialEvent(void) __attribute__((weak));
 #endif // __cplusplus
 
 #else  // !defined(USB_DISABLED)