helix/rev3_4rows, helix/rev3_5rows: Fix build with converters (#21608)

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.

keyboards/helix/rev3_5rows/keymaps/default/oled_display.c

@@ -16,6 +16,8 @@
 
 #include QMK_KEYBOARD_H
 
+#include <stdio.h>
+
 // Defines names for use in layer keycodes and the keymap
 enum layer_names {
   _QWERTY = 0,