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.

source/blender/blenkernel/BKE_appdir.h

@@ -15,6 +15,8 @@
  */
 #pragma once
 
+#include <stddef.h>
+
 /** \file
  * \ingroup bli
  */