kconfig: add missing <stdlib.h> inclusion

This header is needed when using {m,re}alloc(3) and free(3) function family.

Signed-off-by: Arnaud Lacombe <lacombar@gmail.com>
diff --git a/scripts/kconfig/util.c b/scripts/kconfig/util.c
index e07d557..d0b8b23 100644
--- a/scripts/kconfig/util.c
+++ b/scripts/kconfig/util.c
@@ -6,6 +6,7 @@
  */
 
 #include <stdarg.h>
+#include <stdlib.h>
 #include <string.h>
 #include "lkc.h"