4d50e00914
Incorrectly changed in 9e8c037375.
The Grease Pencil `eraser_brush` property is still accessed and set
directly from Python, we cannot remove setting access to the property in
favor of the `AssetWeakReference` without first making other changes.
For now, this patch re-adds the RNA definitions for the property to make
it editable again.
Pull Request: https://projects.blender.org/blender/blender/pulls/135743