Fix serde not in scope error.

[brocaar]

As serde is an optional dependency, the #[serde(default)] must be behind a cfg_attr to make sure it is only applied in case the serde feature is enabled.

[brocaar]

Example error:

    Checking deadpool-redis v0.17.1
error: cannot find attribute `serde` in this scope
  --> /home/brocaar/.cargo/registry/src/index.crates.io-6f17d22bba15001f/deadpool-redis-0.17.1/src/cluster/config.rs:62:7
   |
62 |     #[serde(default)]
   |       ^^^^^

error: could not compile `deadpool-redis` (lib) due to 1 previous error
warning: build failed, waiting for other jobs to finish...

[bikeshedder]

Whopsies! Thank you for the fix! 👍

[bikeshedder]

I just released deadpool-redis 0.17.2 at crates.io which includes this fix:

• 📦 https://crates.io/crates/deadpool-redis/0.17.2