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>
diff --git a/scripts/kconfig/qconf.cc b/scripts/kconfig/qconf.cc
index df274fe..1500c38 100644
--- a/scripts/kconfig/qconf.cc
+++ b/scripts/kconfig/qconf.cc
@@ -6,6 +6,7 @@
 #include <qglobal.h>
 
 #if QT_VERSION < 0x040000
+#include <stddef.h>
 #include <qmainwindow.h>
 #include <qvbox.h>
 #include <qvaluelist.h>