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

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>