audio,ges,rtsp-server: Define GstObjectExt trait rename for docs

The generator does not read Gir.toml from dependencies, missing out on trait renames like ObjectExt turning into GstObjectExt(to prevent clashes withglib's ObjectExt). Renaming through trait_nameis now taken into account ingirthanks to [1], but the renames still need to be availble to all crates referencing this type. Fortunately onlyGst.Object` is affected - other renamed traits are not extended in any of the other crates.