Include <sys/stat.h> to define mode_t in vpf.h

Inspired by:

https://git.alpinelinux.org/cgit/aports/plain/main/varnish/musl-mode_t.patch?h=3.5-stable

bin/varnishlog/varnishlog.c

@@ -39,6 +39,7 @@
 #include <errno.h>
 #include <stdarg.h>
 #include <stdint.h>
+#include <sys/stat.h>
 
 #include "vapi/vsm.h"
 #include "vapi/vsl.h"

lib/libvarnishapi/vut.c

@@ -41,6 +41,7 @@
 #include <string.h>
 #include <signal.h>
 #include <math.h>
+#include <sys/stat.h>
 
 #include "compat/daemon.h"
 #include "vdef.h"