add _cann_ops_custom gitignore (vllm-project#4605)

### What this PR does / why we need it?
add _cann_ops_custom dir to .gitignore

- vLLM version: v0.11.2

Signed-off-by: zzzzwwjj <[email protected]>

Author: zzzzwwjj

.gitignore

@@ -203,5 +203,9 @@ kernel_meta/
 # benchmark results generated by run-performance-benchmarks.sh
 /benchmarks/results/
 
+# _cann_ops_custom generated by build_aclnn.sh
+/vllm_ascend/_cann_ops_custom/*
+!/vllm_ascend/_cann_ops_custom/.gitkeep
+
 # generated by CANN
 fusion_result.json