Filetree-example: Fix sort order.

The debug version of the algorithm in MSVC performs a check
on the less-than function (if a < b, b < a should be false).
Always returning QXmlNodeModelIndex::Precedes fails that.

Task-number: QTBUG-35525

Change-Id: I2ad030284eb76fd07427ffb710b3d3afd63103e5
Reviewed-by: Jan Arve Sæther <jan-arve.saether@digia.com>