Fix import in bench for wasm

[ehuss]

This import was causing annoying unused import errors when checking the standard library for some wasm targets. The problem is that everything here is disabled if it is wasm32, but this import wasn't cfg'd.

[rustbot]

r? @jhpratt

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[jhpratt]

@bors r+ rollup

[bors]

📌 Commit c917373 has been approved by jhpratt

It is now in the queue for this repository.