Bugfix: addIdFilter does not work on category collection when flat catalog is enabled

See it in action on the RSS page when *Top Level Category RSS Feed* is enabled **and** *Flat Catalog Category* is enabled. 

`Mage_Rss_Block_List::CategoriesRssFeed()` calls `addIdFilter($nodeIds)`. `Mage_Catalog_Model_Resource_Category_Flat_Collection::addIdFilter()` in turn calls `$this->addFieldToFilter('entity_id', $condition);`, but `entity_id` has become ambigious because of the join you add on the collection in the observer.

The only I could think of to fix this, was to extend `Mage_Catalog_Model_Resource_Category_Flat_Collection` and change the column reference to `main_table.entity_id`.

Actually, this is a bugfix for the core class which should have used `main_table.entity_id` and not for your extension :)