So far it was needed to declare a new RNA struct to `RNA_access.h` manually.
Since 9b298cf3 we generate a `RNA_prototypes.h` for RNA property
declarations. Now this also includes the RNA struct declarations, so they don't
have to be added manually anymore.

Differential Revision: https://developer.blender.org/D13862

Reviewed by: brecht, campbellbarton