[6.0] 0c8ffd99e Include header to support musl

[Dridi Boukelmoune]

commit 0c8ffd99ed91bc858ab8c50e2955f1dcb85ecdb3
Author: Guillaume Quintard <guillaume at varnish-software.com>
Date:   Thu Apr 26 21:50:38 2018 +0200

    Include header to support musl

diff --git a/bin/varnishtest/vtc_haproxy.c b/bin/varnishtest/vtc_haproxy.c
index 35dd2e898..88162a951 100644
--- a/bin/varnishtest/vtc_haproxy.c
+++ b/bin/varnishtest/vtc_haproxy.c
@@ -33,6 +33,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <sys/stat.h> /* for MUSL (mode_t) */
 #include <unistd.h>
 
 #include "vtc.h"