Fix compile error on OLED disabled

[MiSawa]

When one attempted to build the firmware with OLED feature disabled, following compile errors are thrown.

keyboards/keyball/lib/keyball/keyball.c:69:20: error: ‘format_4d’ defined but not used [-Werror=unused-function]
Compiling: keyboards/keyball/lib/keyball/keyball.c                                                  [ERRORS]
 static const char *format_4d(int8_t d) {
                    ^
keyboards/keyball/lib/keyball/keyball.c:96:13: error: ‘to_1x’ defined but not used [-Werror=unused-function]
 static char to_1x(uint8_t x) {
             ^

These two functions are used only under #ifdef OLED_ENABLE guard, and thus have to be guarded as well.

[koron]

I'll check and merge this after upgrading QMK 0.22.

Thank you!

[koron]

confirmed on local.