Fix clippy::return_self_not_must_use warning

```text
error: missing `#[must_use]` attribute on a method returning `Self`
  --> futures-util/src/stream/select_with_strategy.rs:19:5
   |
19 | /     pub fn toggle(&mut self) -> Self {
20 | |         let old = *self;
21 | |
22 | |         match self {
...  |
27 | |         old
28 | |     }
   | |_____^
   |
```

Author: taiki-e

futures-util/src/stream/select_with_strategy.rs

@@ -16,6 +16,7 @@ pub enum PollNext {
 
 impl PollNext {
     /// Toggle the value and return the old one.
+    #[must_use]
     pub fn toggle(&mut self) -> Self {
         let old = *self;