Use the new FindExiv2 module from ECM.

Summary:
ECM 5.53 ships a modern FindExiv2.cmake module that will be needed
when exiv 0.27 is released.

Test Plan: Build succeeds, tests pass

Reviewers: mgallien

Reviewed By: mgallien

Subscribers: mgallien, kde-frameworks-devel, Baloo

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D17046