Add missing namespace import in MysqlClient

Adds a missing `use` statement for the `PromiseInterface` type to `MysqlClient`.
Without that change, using the API leads to type warnings:

```php
function someMethod(): PromiseInterface {
  return $this->mysql->query(...);
}
// Return value is expected to be '\React\Promise\PromiseInterface', '\React\Mysql\PromiseInterface' returned

src/MysqlClient.php

@@ -8,6 +8,7 @@
 use React\Mysql\Io\Factory;
 use React\Promise\Deferred;
 use React\Promise\Promise;
+use React\Promise\PromiseInterface;
 use React\Socket\ConnectorInterface;
 use React\Stream\ReadableStreamInterface;