Utility functions in UEdGraph_ReferenceViewer are no longer exposed

A number of utility functions are provided in UEdGraph_ReferenceViewer for building menu extenders. This class was originally included in the engine as part of the ReferenceViewer module, but was refactored into an AssetManager plugin. In the course of the refactoring, the class may have been mistakenly moved to the private folder, preventing those functions from being accessed from outside the plugin.

1. Make your project or plugin depend on AssetManagerEditor plugin
2. Attempt to include EdGraph_ReferenceViewer.h
3. It was made private by accident so this will fail. To verify fix just make sure header is in public directory