Third-party: Disable -Werror for flatcc build

[rascani]

Summary

Apple Clang 21 (Xcode 26) elevates -Wimplicit-int-conversion-on-negation and -Wunterminated-string-initialization to errors, which breaks the vendored flatcc build at pprintint.h and grisu3_print.h. The warnings are benign and the fix belongs upstream in dvidelabs/flatcc; disable -Werror locally via FLATCC_ALLOW_WERROR=OFF for both the flatcc_ep external project (host compiler) and the in-tree flatccrt runtime lib.

Test plan

./install_executorch.sh

Authored with Claude.

[pytorch-bot]

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❌ 1 New Failure, 1 Pending

As of commit cf902a0 with merge base 5b89d23 ():

NEW FAILURE - The following job has failed:

• pull / unittest / macos / macos-job (gh)
  extension/pybindings/test/test_pybindings.py::PybindingsTest::test_program_data_separation

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