Fix (studio reported) missing object's parent handling in readfile expand code.

This would prevent loading a parent that would only be referenced by
children during a linking operation.

Looks like this missing bit of code has been there since the stone ages,
it is fairly baffling to find that such critical low-levels mistakes can
survive decades in a codebase...

Note that such fully-indirectly linked parent object is not instantiated
in scene currently, this is fairly bad I think, but kind of a different
issue.

source/blender/blenkernel/intern/object.c

@@ -1041,6 +1041,8 @@ static void object_blend_read_expand(BlendExpander *expander, ID *id)
 
   BLO_expand(expander, ob->data);
 
+  BLO_expand(expander, ob->parent);
+
   /* expand_object_expandModifier() */
   if (ob->modifiers.first) {
     BKE_modifiers_foreach_ID_link(ob, expand_object_expandModifiers, expander);