Fix T46723: Issue with linking to rigs running Blender in new depsgraph mode

This isn't really ideal fix, but currently i can't tell how proxies should work
with the new dependency graph. This backward links are crazy, non-threadsafe
and mindblowing. Need to do some smarter/easier system.

source/blender/depsgraph/intern/depsgraph_build_nodes.cc

@@ -272,6 +272,7 @@ void DepsgraphNodeBuilder::build_scene(Main *bmain, Scene *scene)
 		/* object that this is a proxy for */
 		// XXX: the way that proxies work needs to be completely reviewed!
 		if (ob->proxy) {
+			ob->proxy->proxy_from = ob;
 			build_object(scene, base, ob->proxy);
 		}