[Vala] GLib.Sequence bindings does not bind Sequence.lookup ()

[Stevie Griffiths <stevie craig griffiths gmail com>]

I can work around this using search and a couple of checks I figure,
however I thought it can probably be fixed for everyone (if that isn't
happening already).  I'm not sure how to report the bug properly, if
someone else wants to do it or tell me how it'd make me happy :)

Regards,
Stephen.