IWYU: add missing include for absl::StrCat

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:pipeline:curl -sS "https://raw.githubusercontent.com/bazelbuild/continuous-integration/master/buildkite/bazelci.py?$(date +%s)" -o bazelci.py && python3.6 bazelci.py project_pipeline --file_config=.bazelci/presubmit.yml | tee /dev/tty | buildkite-agent pipeline upload
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:ubuntu: Ubuntu 22.04 LTScurl -sS https://raw.githubusercontent.com/bazelbuild/continuous-integration/master/buildkite/bazelci.py?1712943104 -o bazelci.py && python3 bazelci.py runner --task=ubuntu2204-c++14 --file_config=.bazelci/presubmit.yml
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:ubuntu: Ubuntu 22.04 LTScurl -sS https://raw.githubusercontent.com/bazelbuild/continuous-integration/master/buildkite/bazelci.py?1712943104 -o bazelci.py && python3 bazelci.py runner --task=ubuntu2204-c++17 --file_config=.bazelci/presubmit.yml
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Total Job Run Time: 1m 21s