Fix over/underclock support (#147)

Removed needed header from main.cpp during prior cleanup.  Re-add.

Fixes #146

Author: earlephilhower

cores/rp2040/main.cpp

@@ -20,6 +20,7 @@
 
 #include <Arduino.h>
 #include "RP2040USB.h"
+#include <pico/stdlib.h>
 #include <pico/multicore.h>
 
 RP2040 rp2040;