analog: declare aref variable only if used

Author: pennam

cores/arduino/analog.cpp

@@ -558,7 +558,10 @@ void analogReference(uint8_t mode) {
   R_ADC_Open(&adc1.ctrl, &adc1.cfg);
 }
 
+#if defined(AVCC_MEASURE_PIN)
 static float aref = 0;
+#endif
+
 float analogReference() {
     switch (adc.cfg_extend.adc_vref_control) {
       case ADC_VREF_CONTROL_1_5V_OUTPUT:
@@ -817,4 +820,4 @@ void analogWrite(pin_size_t pinNumber, int value)
 
 FspTimer* __get_timer_for_channel(int channel) {
   return  pwms.get_from_channel(channel)->get_timer();
-}
+}