kconfig: variable argument lists needs `stdarg.h'

Fix build on non GNU based platforms.

Cc: Roman Zippel <[e-mail]>
Signed-off-by: Sam Ravnborg <[e-mail]>

scripts/kconfig/lxdialog/util.c

···
  *  Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
  */
 
+#include <stdarg.h>
+
 #include "dialog.h"
 
 struct dialog_info dlg;