apparmor-libreoffice: try to get the preloaded module from $(dirname $0)

This makes it easier to use uninstalled with

    make -C apparmor/libreoffice
    apparmor/libreoffice/libreoffice normal

Reviewed-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D3633