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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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/20135

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❌ 3 New Failures, 1 Pending

As of commit 82463ae with merge base ac3003e ():

NEW FAILURES - The following jobs have failed:

• pull / test-parakeet-xnnpack-linux / linux-job (gh)
  /pytorch/executorch/examples/models/parakeet/tokenizer_utils.cpp:39:13: error: no matching member function for call to 'rfind'
• Test WebGPU Backend / test-webgpu / test-backend-linux (webgpu, models) / linux-job (gh)
  RuntimeError: Command docker exec -t f87251fe8a741ccb65b8639917434f00a40cf6ed0061c9b24d51862c46394393 /exec failed with exit code 1
• Test WebGPU Backend / test-webgpu / test-backend-linux (webgpu, operators) / linux-job (gh)
  RuntimeError: Command docker exec -t 8ff8961cb57f73cce6504b0abf4802c88f8257f309d373be3df1f1a49800cd99 /exec failed with exit code 1

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