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/confdata.c b/scripts/kconfig/confdata.c
index 4e878dd..ca16ab4 100644
--- a/scripts/kconfig/confdata.c
+++ b/scripts/kconfig/confdata.c
@@ -7,6 +7,7 @@
 #include <ctype.h>
 #include <errno.h>
 #include <fcntl.h>
+#include <stdarg.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>