Fix atexit signature

Fix the atexit signature to be consistent with <stdlib.h>

Author: Ebola-Chan-bot

cores/arduino/Arduino.h

@@ -129,7 +129,7 @@ typedef uint8_t byte;
 void init(void);
 void initVariant(void);
 
-int atexit(void (*func)()) __attribute__((weak));
+int atexit(void (*func)(void)) __attribute__((weak));
 
 void pinMode(uint8_t pin, uint8_t mode);
 void digitalWrite(uint8_t pin, uint8_t val);