use SvXMLExport::GetRelativeReference(), fdo#79305

Change-Id: I1e92a87034cf31470a9d8809f3b543b154c65126
Reviewed-on: https://gerrit.libreoffice.org/10445
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>