Fix build with exiv2 0.28

Summary: Exiv2::Image::AutoPtr has been replaced with Exiv2::Image::UniqePtr in current master

Test Plan: exivextractor test passes, test images are indexed by Baloo

Reviewers: mgallien, astippich, bruns

Reviewed By: bruns

Subscribers: kde-frameworks-devel, Baloo

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D20324