[5.2] 0f37f82 Add necessary include

[PÃ¥l Hermunn Johansen]

commit 0f37f82bdd84ebd5e8b2f0c456183222340e5899
Author: Poul-Henning Kamp <phk at FreeBSD.org>
Date:   Thu Sep 7 11:05:03 2017 +0000

    Add necessary include

diff --git a/bin/varnishd/mgt/mgt_child.c b/bin/varnishd/mgt/mgt_child.c
index 125cc31..ebcaba5 100644
--- a/bin/varnishd/mgt/mgt_child.c
+++ b/bin/varnishd/mgt/mgt_child.c
@@ -38,6 +38,7 @@
 #include <fcntl.h>
 #include <poll.h>
 #include <signal.h>
+#include <stdarg.h>
 #include <stdio.h>
 #include <string.h>
 #include <syslog.h>