As long as it's not merged to varnish-cache, add VRT_DirectorResolve

Ref: https://github.com/varnishcache/varnish-cache/pull/2680

src/vmod_cluster.c

@@ -37,6 +37,32 @@
 
 #include "vcc_cluster_if.h"
 
+/* ------------------------------------------------------------
+ * workaround missing VRT_DirectorResolve
+ * Ref https://github.com/varnishcache/varnish-cache/pull/2680
+ */
+
+#include <cache/cache_director.h>
+
+static VCL_BACKEND
+VRT_DirectorResolve(VRT_CTX, VCL_BACKEND d)
+{
+	VCL_BACKEND d2;
+
+	for (; d != NULL && d->vdir->methods->resolve != NULL; d = d2) {
+		CHECK_OBJ_NOTNULL(d, DIRECTOR_MAGIC);
+		AN(d->vdir);
+		d2 = d->vdir->methods->resolve(ctx, d);
+	}
+	CHECK_OBJ_ORNULL(d, DIRECTOR_MAGIC);
+	if (d != NULL)
+		AN(d->vdir);
+	return (d);
+}
+
+/* ------------------------------------------------------------
+ */
+
 enum resolve_e {
 	_RESOLVE_E_INVALID = 0,
 #define VMODENUM(x) x,