Suppress warnings in the benchmarks for upstream GCC false positives. (backport #810)

[mergify[bot]]

This will fix at least one of the warnings as seen in https://ci.ros2.org/view/nightly/job/nightly_linux_release/3103/gcc/new/

@Crola1702 FYI

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This is an automatic backport of pull request #810 done by Mergify.