Fix T85230: Face Set Boundary automasking not working with dyntopo

Dyntopo does not have Face Sets implemented yet, so the entire mesh is
considered a single Face Set. In that case, the check unique face set
function should always return true.

Reviewed By: JacquesLucke

Maniphest Tasks: T85230

Differential Revision: https://developer.blender.org/D10259

source/blender/editors/sculpt_paint/sculpt.c

@@ -678,7 +678,7 @@ bool SCULPT_vertex_has_unique_face_set(SculptSession *ss, int index)
       return sculpt_check_unique_face_set_in_base_mesh(ss, index);
     }
     case PBVH_BMESH:
-      return false;
+      return true;
     case PBVH_GRIDS: {
       const CCGKey *key = BKE_pbvh_get_grid_key(ss->pbvh);
       const int grid_index = index / key->grid_area;