Fails to build with HDF5 1.12.1

In HDF5 1.12.1, hid_t changed from int to int64_t. This is conflicting with https://gitlab.kitware.com/xdmf/xdmf/-/blob/master/core/XdmfHDF5Controller.hpp#L36, resulting in a failure to build. Changing from int to int64_t allows to compile again.

I guess that to keep compatibility with previous HDF5 version, it should be conditionally defined in XDMF rather than blindly changed.