formula_auditor: add exemption for nghttp2

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.

Library/Homebrew/formula_auditor.rb

@@ -453,6 +453,9 @@ def audit_gcc_dependency
       return unless @core_tap
       return unless Homebrew::SimulateSystem.simulating_or_running_on_linux?
       return unless linux_only_gcc_dep?(formula)
+      # https://github.com/Homebrew/homebrew-core/pull/171634
+      # https://github.com/nghttp2/nghttp2/issues/2194
+      return if formula.name == "nghttp2"
 
       problem "Formulae in homebrew/core should not have a Linux-only dependency on GCC."
     end