Add missing imports for `kotlin.streams.toList` (#3773)

Kotlin `toList` is required for the `stream.toList` extension functions
that are used in this file.

The
[PR](https://github.com/smithy-lang/smithy-rs/pull/2544/files#diff-4444943dae2fce81f218aa052d99a5db6a584f25207d742575ff042c69f7f60a

)
accidentally removed the import statement from `OperationNormalizer.kt`.

Co-authored-by: Fahad Zubair <fahadzub@amazon.com>
Co-authored-by: Zelda Hessler <zhessler@amazon.com>