Fix GHASH preprocessing

[sinui0]

https://github.com/tlsnotary/tlsn/blob/a1deba101b466b0f656ad301da569f0e5c61dd3c/components/universal-hash/src/ghash/ghash_inner/mod.rs#L131

It went unnoticed that we're preprocessing twice as many blocks as necessary, update this to account for the free-squaring trick.