Fix mysql feature also needs futures

[williamdes]

cargo build --manifest-path crates/main/Cargo.toml --release --bin stalwart-mail --no-default-features --features sqlite,mysql,s3,redis

[mdecimus]

Why this change? It is not required by the mysql store implementation.

[williamdes]

Why this change? It is not required by the mysql store implementation.

Run my command and you will see the mysql feature can not build ;)

error[E0432]: unresolved import `futures`
 --> crates/store/src/backend/mysql/read.rs:7:5
  |
7 | use futures::TryStreamExt;
  |     ^^^^^^^ help: a similar path exists: `mysql_async::futures`

error[E0432]: unresolved import `futures`
  --> crates/store/src/backend/mysql/write.rs:10:5
   |
10 | use futures::TryStreamExt;
   |     ^^^^^^^ help: a similar path exists: `mysql_async::futures`