writerfilter/source: change directory dependencies to .../ooxml/.dir

fixes build errors like:
  ./workdir/CustomTarget/writerfilter/source/ooxml/model_preprocessed.xml: No such file or directory

Change-Id: I1ee4e6247e9b87548a0314914fd3fc6499783ea6
Reviewed-on: https://gerrit.libreoffice.org/10008
Tested-by: Michael Stahl <mstahl@redhat.com>
Reviewed-by: Michael Stahl <mstahl@redhat.com>