Fix specialized light tree building for distant lights

On a user level this fixes configuration when a spot light is
linked to an object, and a sun light is not linked to anything.
It used to be making non-linked receivers to be very noisy.

This is because the distant light did not update the node's
light linking settings when they are added to the node.

A simple demo file will be added to the tests suit as
light_link_distant_tree.blend.

Pull Request: https://projects.blender.org/blender/blender/pulls/108311

intern/cycles/scene/light_tree.h

@@ -276,6 +276,7 @@ struct LightTreeNode {
   __forceinline void add(const LightTreeEmitter &emitter)
   {
     measure.add(emitter.measure);
+    light_link.add(emitter.light_set_membership);
   }
 
   __forceinline Leaf &get_leaf()