Ignore clippy::mixed_attributes_style lint

```
error: item has both inner and outer attributes
   --> futures/src/lib.rs:148:1
    |
148 | / #[cfg(feature = "executor")]
149 | | #[cfg_attr(docsrs, doc(cfg(feature = "executor")))]
150 | | pub mod executor {
151 | |     //! Built-in executors and related tools.
...   |
187 | |     //! [`spawn_obj`]: https://docs.rs/futures/0.3/futures/task/trait.Spawn.html#tymethod.spawn_obj
188 | |     //! [`spawn_local_obj`]: https://docs.rs/futures/0.3/futures/task/trait.LocalSpawn.html#tymethod.spawn_local_obj
    | |____________________________________________________________________________________________________________________^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#mixed_attributes_style
    = note: `-D clippy::mixed-attributes-style` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::mixed_attributes_style)]`

error: item has both inner and outer attributes
   --> futures/src/lib.rs:200:1
    |
200 | / #[cfg(feature = "compat")]
201 | | #[cfg_attr(docsrs, doc(cfg(feature = "compat")))]
202 | | pub mod compat {
203 | |     //! Interop between `futures` 0.1 and 0.3.
204 | |     //!
205 | |     //! This module is only available when the `compat` feature of this
206 | |     //! library is activated.
    | |_____________________________^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#mixed_attributes_style
```

Author: taiki-e

futures/src/lib.rs

@@ -145,6 +145,7 @@ pub use futures_util::lock;
 #[doc(inline)]
 pub use futures_util::io;
 
+#[allow(clippy::mixed_attributes_style)] // https://github.com/rust-lang/rust-clippy/issues/12435
 #[cfg(feature = "executor")]
 #[cfg_attr(docsrs, doc(cfg(feature = "executor")))]
 pub mod executor {
@@ -197,6 +198,7 @@ pub mod executor {
     pub use futures_executor::{ThreadPool, ThreadPoolBuilder};
 }
 
+#[allow(clippy::mixed_attributes_style)] // https://github.com/rust-lang/rust-clippy/issues/12435
 #[cfg(feature = "compat")]
 #[cfg_attr(docsrs, doc(cfg(feature = "compat")))]
 pub mod compat {