Update the geometry predicate for OSM

So far, the code supported getting the geometries via `geo:hasGeometry`
(old osm2rdf style) and via `wdt:P625` (Wikidata style). Since November,
the TTLs provided on https:/osm2rdf.cs.uni-freiburg.de provide the
geometries via geo:hasGeometry/geo:asWKT, which is now also supported.

TODO: The selection between is currently hard-coded, based on the name
of the backend. We should find a more elegant and robust mechanism for
this.