Update trek.h to add __unused attribute

* Reapply following commit

    commit 1b12f0c0fe3803aa6638a285c64320839b293977
    Author: Kenji Rikitake <kenji.rikitake@acm.org>
    Date:   Sat May 3 09:48:15 2014 +0900

        Add __unused definition to trek.h