Fix hardness not resetting when using reset brush

Just add it to the list or properties to reset from DNA_brush_defaults

Reviewed By: jbakker

Differential Revision: https://developer.blender.org/D6992

source/blender/blenkernel/intern/brush.c

@@ -78,6 +78,7 @@ static void brush_defaults(Brush *brush)
   FROM_DEFAULT(weight);
   FROM_DEFAULT(size);
   FROM_DEFAULT(alpha);
+  FROM_DEFAULT(hardness);
   FROM_DEFAULT(autosmooth_factor);
   FROM_DEFAULT(topology_rake_factor);
   FROM_DEFAULT(crease_pinch_factor);