util/codegen: permit running in directories without copyright headers

It broke in our corp repo that lacks copyright headers.

Change-Id: Iafc433e6b6affe83b45477899455527658dc4f12
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f