adjust to vmod_priv methods

Ref:
https://github.com/varnishcache/varnish-cache/commit/681c11998ec56dc4460a30585210f95a7850ea3b

src/vmod_globalvar.c

@@ -99,6 +99,11 @@ task_unref_var(void *ptr) {
 	unref_var((struct vmod_globalvar_var *) ptr);
 }
 
+static const struct vmod_priv_methods priv_task_methods[1] = {{
+		.magic = VMOD_PRIV_METHODS_MAGIC,
+		.type = "vmod_globalvar_priv_task",
+		.fini = task_unref_var
+}};
 
 static const void *
 task_ref_var(VRT_CTX, struct vmod_globalvar_var *v, unsigned magic) {
@@ -119,7 +124,7 @@ task_ref_var(VRT_CTX, struct vmod_globalvar_var *v, unsigned magic) {
 	if (t->priv == NULL) {
 		ref_var(v);
 		t->priv = v;
-		t->free = task_unref_var;
+		t->methods = priv_task_methods;
 	}
 
 	assert(t->priv == v);