[master] d8041f13a include <stddef.h> to pick up __GNUC_PREREQ__ if possible.

[Poul-Henning Kamp]

commit d8041f13ac2255df2040af1ccc626555c39066e1
Author: Poul-Henning Kamp <phk at FreeBSD.org>
Date:   Mon Mar 29 08:05:42 2021 +0000

    include <stddef.h> to pick up __GNUC_PREREQ__ if possible.

diff --git a/include/vdef.h b/include/vdef.h
index a9111feb6..5ba66828e 100644
--- a/include/vdef.h
+++ b/include/vdef.h
@@ -37,6 +37,8 @@
  * This file should always be the first non <...> include in a .c file.
  */
 
+#include <stddef.h>
+
 #ifdef VDEF_H_INCLUDED
 #  error "vdef.h included multiple times"
 #endif