[GEOS-6654] Refer to right Xlink XSD in WFS

In WFS 2.0 the wrong Xlink XSD was linked, instead of using the original
W3 Xlink, the old and deprecated OGC Xlink was used.  This change should
not impact further functionality of the WFS module.
Merge pull request #764 from Geodan/GEOS-6654

[GEOS-6654] Refer to right Xlink XSD in WFS