Flag enums are already treated as a special kind of enum within
`GIInfoType`, so it would be tidier to give it its own `GType` too, with
a subtype relation to `GI_TYPE_ENUM_INFO`.
This will simplify implementing `GI_IS_ENUM_INFO` in a following commit
too.
Signed-off-by: Philip Withnall <pwithnall@gnome.org>
Helps: #3216
