formula_auditor: add exemption for nghttp2

[carlocab]

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Needed for Homebrew/homebrew-core#171634 due to nghttp2/nghttp2#2194.

I've avoided adding an allowlist for now because I really don't want to
see this list grow.

[MikeMcQuaid]

Makes sense!

[MikeMcQuaid]

Thanks again @carlocab!