This used to work by accident for the particular textbox, but broke with
commit 38f2b8b3 (tdf#88811
SwXText::convertToTextFrame: handle shapes anchored to us, 2015-02-03),
so write ZOrder explicitly, like other filters already do.

Change-Id: Ie8f11f4ec77707fb5aec7a597544d589a99bf6ca
Reviewed-on: https://gerrit.libreoffice.org/41921Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>