[Misc] Add benchmark results into .gitignore (#4200)

### What this PR does / why we need it?
Add benchmark results into `.gitignore`

- vLLM version: v0.11.0
- vLLM main:
vllm-project/vllm@2918c1b

Signed-off-by: shen-shanshan <[email protected]>

Author: shen-shanshan

.gitignore

@@ -200,5 +200,8 @@ kernel_meta/
 /vllm_ascend/_build_info.py
 /vllm_ascend/include/
 
+# benchmark results generated by run-performance-benchmarks.sh
+/benchmarks/results/
+
 # generated by CANN
 fusion_result.json