Fixed unused argument warning

cores/arduino/main.cpp

@@ -19,8 +19,8 @@
 
 #include <Arduino.h>
 
-//Declared weak in Arduino.h to allow user redefinitions.
-int atexit(void (*func)()) { return 0; }
+// Declared weak in Arduino.h to allow user redefinitions.
+int atexit(void (* /*func*/ )()) { return 0; }
 
 // Weak empty variant initialization function.
 // May be redefined by variant files.