KisExiv2IoDevice: fix types on 32-bit host with exiv2 0.28
upstream commit https://github.com/Exiv2/exiv2/commit/813566526c9ecdc92fc55c7a7d4de2d8e73e125a now uses int64_t instead of long
this worked for 64-bit ILP64 since long was int64_t, but it does not match on 32-bit platforms