Cleanup: Add missing PBVH forward declaration

Pull Request: https://projects.blender.org/blender/blender/pulls/123039

source/blender/editors/sculpt_paint/mesh_brush_common.hh

@@ -28,6 +28,7 @@
 struct Brush;
 struct Mesh;
 struct Object;
+struct PBVH;
 struct PBVHNode;
 struct Sculpt;
 struct SculptSession;