Add missing function

After the introduction of the following commit in upstream Packer:

commit 90a57c411fba78acc3ef1f565b46f2cf00e96a83
Author: Julian Phillips <julian@quantumfyre.co.uk>
Date:   Sun Jan 19 18:32:44 2014 +0000

    Expand Artifact API to expose build state

    In order that something consuming an artifact can have access to extra
    builder specific data add the State method which allows the caller to
    ask for arbitary values by name.

A function called State must be defined when implementing the Artifact
API.