[CI] Add flashinfer.py to attention test source deps (#38792)

Signed-off-by: Stefano Castagnetta <scastagnetta@nvidia.com>
Co-authored-by: Luka Govedič <ProExpertProg@users.noreply.github.com>

.buildkite/test_areas/kernels.yaml

@@ -29,6 +29,7 @@ steps:
   - vllm/v1/attention
     # TODO: remove this dependency (https://github.com/vllm-project/vllm/issues/32267)
   - vllm/model_executor/layers/attention
+  - vllm/utils/flashinfer.py
   - tests/kernels/attention
   commands:
     - pytest -v -s kernels/attention --shard-id=$$BUILDKITE_PARALLEL_JOB --num-shards=$$BUILDKITE_PARALLEL_JOB_COUNT