Work around exiv2 still using std::auto_ptr
std::auto_ptr is being removed when building with newer C++ standards. Our consumers so far work around that by reducing the C++ standard for files or targets using exiv2. That however no longer works with Qt 6, which insists on C++17 at least.
Instead, we now make std::auto_ptr visible again in libc++ for exiv2 users.
This should fix the FreeBSD Qt 6 build of KFileMetaData.