Replace chained O(U)StringBuffer::append() in PivotLayoutTreeListData.cxx

Change-Id: Ia1cc2f32e038bca2421cc5e9aca3b4f15d7c8432
Reviewed-on: https://gerrit.libreoffice.org/22828Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>