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>

scripts/kconfig/util.c

@@ -6,6 +6,7 @@
  */
 
 #include <stdarg.h>
+#include <stdlib.h>
 #include <string.h>
 #include "lkc.h"