graphics/digikam: make compatible with exiv2 0.28
NOTE: Upstream has just tagged digikam 8.1.0 which
should also work with exiv2 0.28, see here:
https://invent.kde.org/graphics/digikam/-/blob/v8.1.0/NEWS
But the web site does not yet mention 8.1.0 and the release
tarball is not available, so upgrading might be premature.
PR: 272311