Use ndirector, not serial in DNS director.

The test case for this doesn't trigger, but the failing VCL (when
given in a file with -f) looks like:

director squid round-robin {
	 { .backend = { .host = "127.0.0.1"; .port = "3131"; } }
}

director dnsdir dns {
	 .list = {
	       "201.7.184.0"/32;
	 }
}

sub vcl_recv {
    set req.backend = dnsdir;
}

Fixes #1295

lib/libvcl/vcc_dir_dns.c

@@ -87,7 +87,7 @@ print_backend(struct vcc *tl,
 	AN(vsb);
 	tl->fb = vsb;
 	Fc(tl, 0, "\t{ .host = VGC_backend_%s },\n",vgcname);
-	Fh(tl, 1, "\n#define VGC_backend_%s %d\n", vgcname, serial);
+	Fh(tl, 1, "\n#define VGC_backend_%s %d\n", vgcname, tl->ndirector);
 
 	Fb(tl, 0, "\nstatic const struct vrt_backend vgc_dir_priv_%s = {\n",
 	    vgcname);