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.
,-----------------------------------------. ,-----------------------------------------.
| Tab | Q | W | E | R | T | | Y | U | I | O | P | Bksp |
|------+------+------+------+------+------| |------+------+------+------+------+------|
| Ctrl | A | S | D | F | G | | H | J | K | L | ; | ' |
|------+------+------+------+------+------| |------+------+------+------+------+------|
| Shift| Z | X | C | V | B | | N | M | , | . | / |Enter |
|------+------+------+------+------+------+-------------+------+------+------+------+------+------|
|Adjust| Esc | Alt | GUI | EISU |Lower |Space |Space |Raise | KANA | Left | Down | Up |Right |
`-------------------------------------------------------------------------------------------------'