macros/class: Propagate cfg annotations to COM class methods

We have a very peculiar workaround for DXC (pending upstream fix
[microsoft/DirectXShaderCompiler#3793]) that requires us to
conditionally provide function implementations for extra vtable
"spacers": https://github.com/Traverse-Research/hassle-rs/blob/f5a090c70bbcf66f3bafd3d549716a414e873838/src/wrapper.rs#L130-L139.
The spacers are conditionally defined in:
https://github.com/Traverse-Research/hassle-rs/blob/f5a090c70bbcf66f3bafd3d549716a414e873838/src/unknown.rs.

These need to be forwarded otherwise the initialization of these vtable
members will happen unconditionally even when they don't exist:
https://github.com/Traverse-Research/hassle-rs/actions/runs/1777800733

Fixes microsoft#166
[microsoft/DirectXShaderCompiler#3793]: microsoft/DirectXShaderCompiler#3793

macros/support/src/class/class.rs

@@ -625,7 +625,9 @@ impl Interface {
                 }
             };
             let field_name = Ident::new(&crate::utils::snake_to_camel(&original_name.to_string()), proc_macro2::Span::call_site());
+            let attrs = &m.item.attrs;
             quote! {
+                #(#attrs)*
                 #field_name: {
                     #method
                     #name

src/production.rs

@@ -1,5 +1,4 @@
 mod class;
-#[cfg(windows)]
 #[doc(hidden)]
 #[cfg(windows)]
 pub mod registration;