`--with-derive-custom-struct` doesn't apply to `__BindgenBitfieldUnit`

[BennoLossin]

Invoking bindgen with --with-derive-custom-struct .*=Foo does not add #[derive(Foo)] to __BindgenBitfieldUnit.

I'd like all structs (& unions) to be marked with the derive macro, as it only implements the trait when all fields also implement it.

My current solution is to just manually implement the trait for __BindgenBitfieldUnit, which is fine, but not stable when you decide to rework how bitfields are represented.

[BennoLossin]

A bit more context: I found this behavior when trying to add #[derive(MaybeZeroable)] to all structs in the bindings crate of Rust-for-Linux.

Cc @ojeda