Fix calc_MS_Works_Document extensions

Change-Id: Icbf087464997561b09c7b43c2269eea2c8c603bb
(cherry picked from commit e5ac048af125e82829d353ec031c99e96572e2c0)
Reviewed-on: https://gerrit.libreoffice.org/10728
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>