Fix iOS app preprocessor flags

[jackzhxng]

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[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/10594

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⏳ 6 Pending, 3 Unrelated Failures

As of commit 43d2a91 with merge base b2eac7d ():

FLAKY - The following jobs failed but were likely due to flakiness present on trunk:

• pull / test-eval_llama-mmlu-linux / linux-job (gh) (detected as infra flaky with no log or failing log classifier)
• pull / test-eval_llama-wikitext-linux / linux-job (gh) (detected as infra flaky with no log or failing log classifier)
• pull / test-llama_runner_eager-linux / linux-job (gh) (detected as infra flaky with no log or failing log classifier)

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