Blender Git Loki

Kernel: include header file in BKE_appdir.h defining size_t

In `BKE_appdir.h`, include `<stddef.h>` as that defines `size_t`. This
follows the "include what you use" principle, and makes it possible to
use `BKE_appdir.h` without having to bother with its dependencies.

No functional changes.