Bump tokenizers submodule to fix sentencepiece GCC 15 build

[rascani]

Summary

Updates extension/llm/tokenizers to include
meta-pytorch/tokenizers#193, which bumps the sentencepiece submodule to pick up a missing #include <cstdint> (google/sentencepiece#1109).

Without this, pytorch_tokenizers fails to compile inside the executorch-ubuntu-26.04-gcc15 docker image, blocking the RISC-V baremetal CI (#19917).

Test plan

CI

[pytorch-bot]

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❌ 4 New Failures, 1 Unclassified Failure

As of commit bc3e7cd with merge base ac3003e ():

NEW FAILURES - The following jobs have failed:

• pull / test-lora-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 7f06869c119439f259e02815a7412a485f435b2f120f9bdd8fb1b39fb2148a10 /exec failed with exit code 1
• pull / test-qnn-python-imports-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 0dc3b11ff96c56dd20ea6537639a6a089fd639b1ed015a16e0a13eb62e21ffcd /exec failed with exit code 92
• Test WebGPU Backend / test-webgpu / test-backend-linux (webgpu, models) / linux-job (gh)
  RuntimeError: Command docker exec -t 0210257beedf2610836b7a42975ba52f8de23dc83f991f87b82cd8292e182f03 /exec failed with exit code 1
• Test WebGPU Backend / test-webgpu / test-backend-linux (webgpu, operators) / linux-job (gh)
  RuntimeError: Command docker exec -t 41d09b579d2d741e8a8d2d2464321f8e43a0de66a1d934c6026dc953ed3dacc8 /exec failed with exit code 1

UNCLASSIFIED FAILURE - DrCI could not classify the following job because the workflow did not run on the merge base. The failure may be pre-existing on trunk or introduced by this PR:

• MLX / test-mlx / test-mlx (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 4

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[rascani]

Failures unrelated.