exiv2 0.28.0 was released a few days ago, but kfilemetadata's code is incompatible and does not build with exiv2 0.28.0. See also: https://bugs.gentoo.org/906090
*** Bug 469635 has been marked as a duplicate of this bug. ***
I've asked upstream for a portability guide https://github.com/Exiv2/exiv2/issues/2630