kconfig: fix a compiliation error when using make xconfig

When using make xconfig, the following compilation error appears :
   /usr/include/qt3/qvaluelist.h:427:13: error: ‘ptrdiff_t’ does not name a type
Including stddef.h in scripts/kconfig/qconf.cc permits to avoid this error.

Signed-off-by: Tiana Rakotovao Andriamahefa <rkmahefa@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>