mkenums: Support public/private trigraph again

Hi! Support for the /*< private >*/ and /*< public >*/ trigraphs in glib-mkenums was added in 9ba17d51 but this change apparently got lost in the Python conversion of this helper script.

This patch adds the change back, and thereby stops the build-log spam as well as honoring the "private" annotation request by skipping the enums coming after that annotation. See the original commit for a more detailed description of this change.