xmlsecurity: HIG fixes

Change-Id: I1472a6b23d0dc3fa9047bf197e9330c06c82f544
Reviewed-on: https://gerrit.libreoffice.org/13328
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>