Make const variables const

Change-Id: Ib29fa559028327ffb8835daf271da0769fecaa2a
Reviewed-on: https://gerrit.libreoffice.org/12478
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>