[packagekit] Small API fixup

[Richard Hughes]

I've just committed this:

commit c0f93fb6119be8fe821a9f0191a2e655dc529d30
Author: Richard Hughes <richard at hughsie.com>
Date:   Mon Jun 17 14:48:53 2013 +0100

    Make pk_backend_job_files() accept an array of strings

    At the moment C/C++ backends have to convert from char** to char* which the
    daemon promptly converts back to char**.

    There's no point penalising all the backends for the spawned
backends sake and
    doing all the splitting and joining hurts performance for packages
that have a
    lot of files.

A couple of the non-spawned backends will need a tiny patch in the
next couple of weeks. I'll fix dummy and zif now.

Thanks,

Richard.