Use [[fallthrough]]; to mark whished fallthroughs

- closes https://github.com/assimp/assimp/issues/4654

code/AssetLib/3DS/3DSConverter.cpp

@@ -262,7 +262,8 @@ void Discreet3DSImporter::ConvertMaterial(D3DS::Material &oldMat,
         unsigned int iWire = 1;
         mat.AddProperty<int>((int *)&iWire, 1, AI_MATKEY_ENABLE_WIREFRAME);
     }
-// fallthrough
+    [[fallthrough]];
+
     case D3DS::Discreet3DS::Gouraud:
         eShading = aiShadingMode_Gouraud;
         break;