libodfgen produces strings in utf8 encoding

Change-Id: Iad17f4b4786473205669bd5b080b0a64d4a6a441
Reviewed-on: https://gerrit.libreoffice.org/7455
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>
(cherry picked from commit 9e5e7c9b556bf9386fb5ad3ecd28b3d43aeeb393)
Reviewed-on: https://gerrit.libreoffice.org/7456