kconfig: add missing <stdarg.h> inclusion

This header is needed when using va_{start,end,copy}(3) functions family.

Signed-off-by: Arnaud Lacombe <lacombar@gmail.com>
diff --git a/scripts/kconfig/util.c b/scripts/kconfig/util.c
index 6330cc8..e07d557 100644
--- a/scripts/kconfig/util.c
+++ b/scripts/kconfig/util.c
@@ -5,6 +5,7 @@
  * Released under the terms of the GNU GPL v2.0.
  */
 
+#include <stdarg.h>
 #include <string.h>
 #include "lkc.h"