remove pico/stdlib.h include to fix pico build

[tahnok]

Using the current versions of platformio libraries available today, I can't run platformio run -e pico without hitting

Compiling .pio/build/pico/src/platform/pico/stdlib.c.o
In file included from src/platform/pico/sleep.c:9:
/home/wes/.platformio/packages/framework-arduino-mbed/cores/arduino/mbed/targets/TARGET_RASPBERRYPI/TARGET_RP2040/pico-sdk/common/pico_stdlib/include/pico/stdlib.h:11:10: fatal error: pico/stdio.h: No such file or directory
   11 | #include "pico/stdio.h"

I found the fix in the fork from @nvts8a in https://github.com/nvts8a/libros/commits/58ede0950a810d9dc43a34748de46154c90a89f4