Cleanup: remove redundant struct declarations

source/blender/blenkernel/BKE_gpencil.h

@@ -36,7 +36,6 @@ struct ListBase;
 struct MDeformVert;
 struct Main;
 struct Material;
-struct MaterialGPencilStyle;
 struct Object;
 struct Scene;
 struct SpaceImage;
@@ -46,7 +45,6 @@ struct bDeformGroup;
 struct bGPDframe;
 struct bGPDlayer;
 struct bGPDlayer_Mask;
-struct bGPDspoint;
 struct bGPDstroke;
 struct bGPDcurve;
 struct bGPdata;