Use different vtable padding on Windows

Author: Alan Jeffrey

src/rust.rs

@@ -459,7 +459,7 @@ impl<T> CustomAutoRooter<T> {
 /// so provide trace logic via vftable when creating an object on Rust side.
 unsafe trait CustomAutoTraceable: Sized {
     const vftable: CustomAutoRooterVFTable = CustomAutoRooterVFTable {
-        padding: [1,3],
+        padding: CustomAutoRooterVFTable::PADDING,
         trace: Self::trace,
     };