Ignore clippy::redundant_clone lint in example

rust-lang/rust-clippy#10545

```
warning: redundant clone
  --> examples/const_fn/src/lib.rs:25:38
   |
25 |     let mut function = const_function.clone();
   |                                      ^^^^^^^^ help: remove this
   |
note: cloned value is neither consumed nor mutated
  --> examples/const_fn/src/lib.rs:25:24
   |
25 |     let mut function = const_function.clone();
   |                        ^^^^^^^^^^^^^^^^^^^^^^
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#redundant_clone
   = note: `#[warn(clippy::redundant_clone)]` on by default
```

examples/const_fn/src/lib.rs

@@ -22,6 +22,7 @@ pub fn const_fn(args: TokenStream, function: TokenStream) -> TokenStream {
         .into();
     }
 
+    #[allow(clippy::redundant_clone)] // https://github.com/rust-lang/rust-clippy/issues/10545
     let mut function = const_function.clone();
     function.sig.constness = None;