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ARROW-9120: [C++] Do not suppress linting on files with "codegen" in their name

This is an artifact of the old kernels code where there was a generated file that was not linted.

Closes #7424 from wesm/ARROW-9120

Authored-by: Wes McKinney <wesm@apache.org>
Signed-off-by: Wes McKinney <wesm@apache.org>