Fix: Missing const specifier for curve field input

Mistake in 000e722c7d, which probably made the viewer node
auto-domain detection behave differently when the special case was used.

source/blender/nodes/geometry/nodes/node_geo_curve_topology_points_of_curve.cc

@@ -219,7 +219,7 @@ class CurveStartPointInput final : public bke::CurvesFieldInput {
     return false;
   }
 
-  std::optional<eAttrDomain> preferred_domain(const bke::CurvesGeometry & /*curves*/)
+  std::optional<eAttrDomain> preferred_domain(const bke::CurvesGeometry & /*curves*/) const final
   {
     return ATTR_DOMAIN_CURVE;
   }