Fix FindLibExiv2 version detection from header
There is an additional "U" character now, see https://github.com/Exiv2/exiv2/commit/b3f2ab59881606954c906292ead9757889838cfc
There is an additional "U" character now, see https://github.com/Exiv2/exiv2/commit/b3f2ab59881606954c906292ead9757889838cfc