Fix building against exiv2-0.27
Summary:
Copied FindLibExiv2.cmake from ECM 5.53.0 until we raise min version.
Upstream advises to include single exiv2/exiv2.hpp over separate headers
that are in the process of being refactored.
BUG: 402644
Test Plan: Tested build with exiv2-0.26 and 0.27
Reviewers: KDE Applications, cgilles, aacid, bruns
Reviewed By: bruns
Subscribers: ngraham, bruns, heikobecker
Differential Revision: https://phabricator.kde.org/D17799