Fix llama runner on MacOS

[mergennachin]

Summary:

Fixes two things:

• There's a compilation error when building the tests due to missing header. Make sure it works via test_llama.sh
• rpath isn't set correctly on MacOS

Test Plan:

cmake --preset llm -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=cmake-out

cmake --build cmake-out -j16 --target install --config Release

cmake -DCMAKE_INSTALL_PREFIX=cmake-out \
	-DBUILD_TESTING=OFF \
	-DCMAKE_BUILD_TYPE=Release \
	-Bcmake-out/examples/models/llama \
	examples/models/llama

cmake --build cmake-out/examples/models/llama -j16 --config Release

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/13869

• 📄 Preview Python docs built from this PR

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❗ 1 Active SEVs

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• ROCm MI2xx CI/CD workflows failing due to : download from https://api.github.com/repos/pytorch/pytorch timed out.

❌ 1 New Failure, 15 Pending, 2 Unrelated Failures

As of commit 853b6a2 with merge base e38c077 ():

NEW FAILURE - The following job has failed:

• Build documentation / build (buck2) / Build doc (gh)
  At least one of the pre-conditions you specified did not hold

BROKEN TRUNK - The following jobs failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / unittest-nxp-neutron / linux-job (gh) (trunk failure)
  test_split_group_convolution__applied_by_default
• trunk / test-huggingface-transformers-coreml (qwen3-1.7b|xnnpack|--quantize) / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 139

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[facebook-github-bot]

@mergennachin has imported this pull request. If you are a Meta employee, you can view this in D81497817.

[facebook-github-bot]

@mergennachin has imported this pull request. If you are a Meta employee, you can view this in D81497817.

[jackzhxng]

LGTM (I may have just merged a similar cmakelist change but feel free to override)