Avoids pedantic errors from modern C compilers.

Fixes #70769

Change-Id: Ibe0d9960e998eb0346b97d7bd69eb7de0d0e6c60
Reviewed-on: https://go-review.googlesource.com/c/go/+/635095


Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Mauri de Souza Meneguzzo <mauri870@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Commit-Queue: Ian Lance Taylor <iant@google.com>
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