docs: Add Eina.Value extern to eina_types.eot
Eina.Values are built-in eolian types, accessed through any_value and
any_value_ptr. However, these types cannot be used in doc references.
Adding a placeholder extern struct Eina.Value causes no harm, and will allow
referencing the type from EO docs later on.
In C#, Eina.Value is defined in the manual binding code so the doc reference
resolves to a valid type.
Reviewed-by: Marcel Hollerbach <firstname.lastname@example.org>
Differential Revision: https://phab.enlightenment.org/D8510