Fix temporary value lifetime in serialize_struct

In the 2024 edition of Rust, `serde`s macros for `serialize_with` can
lead to a temporary lifetime error such as:

```
error[E0716]: temporary value dropped while borrowed
 --> my-binary/src/main.rs:6:10
  |
6 | #[derive(MyDerive)]
  |          ^^^^^^^-
  |          |      |
  |          |      temporary value is freed at the end of this statement
  |          creates a temporary value which is freed while still in use
  |          borrow later used by call
  |          in this derive macro expansion
  |
 ::: /private/tmp/life/my-project/my-macro/src/lib.rs:6:1
  |
6 | pub fn my_derive(_input: TokenStream) -> TokenStream {
  | ---------------------------------------------------- in this expansion of `#[derive(MyDerive)]`
  |
  = note: consider using a `let` binding to create a longer lived value
```

This is because the macro code takes a reference to struct inside of a
block, which then goes out of scope when `serde` passes it to a
function.

To resolve this, we move the reference to outside of the block, to
ensure that the lifetime extends into the function call.

Signed-off-by: Andrew V. Teylu <andrew.teylu@vector.com>

serde_derive/src/ser.rs

@@ -1235,7 +1235,7 @@ fn wrap_serialize_with(
         #serialize_with(#(#self_var.values.#field_access, )* #serializer_var)
     };
 
-    quote!({
+    quote!(&{
         #[doc(hidden)]
         struct __SerializeWith #wrapper_impl_generics #where_clause {
             values: (#(&'__a #field_tys, )*),
@@ -1252,7 +1252,7 @@ fn wrap_serialize_with(
             }
         }
 
-        &__SerializeWith {
+        __SerializeWith {
             values: (#(#field_exprs, )*),
             phantom: _serde::__private::PhantomData::<#this_type #ty_generics>,
         }