Silence warning in greasepencil-object branch

We don't have warnings in 2.8 because of include order.
But headers shouldn't depend on that for warnings, really.

source/blender/blenkernel/BKE_dynamicpaint.h

@@ -28,6 +28,8 @@
  */
 
 struct Depsgraph;
+struct DynamicPaintCanvasSettings;
+struct DynamicPaintModifierData;
 struct Main;
 struct Scene;
 struct ViewLayer;