glib-mkenums: fix parsing of /*< flags >*/ annotation

Fixes get_type function generation for:

 - GMountMountFlags
 - GDriveStartFlags
 - GResourceLookupFlags
 - GSocketMsgFlags
 - GTlsDatabaseVerifyFlags
 - GTestDBusFlags

which were registered as enum types before, which broke
some unit tests.

Problem is that the flags annotation has no value, so
options.get('flags') would always return None even if
it was present.

https://bugzilla.gnome.org/show_bug.cgi?id=779332