[annotations] Fix return type of gtk_tree_model_filter_new.

The return type of gtk_tree_model_filter_new is wrong.

That means that none of the methods introduced in GtkTreeModelFilter for that object are available to the caller.

Without the patch, this causes a small problem in guile-gi otherwise.

See also https://github.com/spk121/guile-gi/issues/99

This patch fixes the annotated return type of gtk_tree_model_filter_new.