Include stdlib in fontutils (#11251)

When `REPRAP_DISCOUNT_FULL_GRAPHIC_SMART_CONTROLLER` is defined and Re-ARM build is attempted, `u8g_fontutf8.cpp` includes `fontutils.h` which uses `abs()`.

However, `abs()` is defined in `stdlib.h` which is not included in `fontutils.h`, so compile fails with an error.

Include `stdlib.h` in `fontutils.h` so that `abs()` is defined before `u8g_fontutf8.cpp` needs to use it, resolving the compile error.

Signed-off-by: Jamie Bainbridge <jamie.bainbridge@gmail.com>