The code for Helix rev3 boards could not be built with, e.g.,
`CONVERT_TO=rp2040_ce` due to invalid `#include <stdio.h>` in the
keyboard level `config.h` (apparently the AVR version of that file
contains guards against usage in assembly code, which was hiding the
bug). Move `#include <stdio.h>` to the C sources which need it.