fix odf validation error

This is the import only change for older release branches.

New elements must not be exported into the chart namespace.

Change-Id: I295d61224e255dff04ea8564b01a5a438876afc1
Reviewed-on: https://gerrit.libreoffice.org/8579
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>