basic support for non-avr, non-usb boards

This is the smallest change to make Kaleidoscope compile for
nRF52 BLE boards.

https://github.com/wez/KaleidoscopeKeyboards
has more code and build machinery for my proof of concept for using
Kaleidoscope as the driver for a keyboard using the new nRF52 based
board from Adafruit.

src/Kaleidoscope.cpp

@@ -9,7 +9,9 @@ Kaleidoscope_::Kaleidoscope_(void) {
 
 void
 Kaleidoscope_::setup(void) {
+#ifdef __AVR__
     wdt_disable();
+#endif
     delay(100);
     Keyboard.begin();
     KeyboardHardware.setup();

src/Kaleidoscope.h

@@ -18,7 +18,9 @@ void setup();
 
 #include <stdio.h>
 #include <math.h>
+#ifdef __AVR__
 #include <avr/wdt.h>
+#endif
 
 #include KALEIDOSCOPE_HARDWARE_H
 #include "key_events.h"

src/key_events.h

@@ -1,6 +1,8 @@
 #pragma once
 #include <Arduino.h>
+#ifdef USBCON
 #include "KeyboardioHID.h"
+#endif
 
 #include KALEIDOSCOPE_HARDWARE_H
 #include "key_defs.h"