convert the LINK we use here to std::function, since LINK
does not currently handle std::unique_ptr

Change-Id: I9df80352e612445e5f5ca513d7d4196d65589778
Reviewed-on: https://gerrit.libreoffice.org/59804
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>