Guard hip bf16 include for ROCm

[volkermauel]

Summary

• check for amd_hip_bf16.h before including hip_bf16.h to avoid missing header build failure on ROCm

Testing

• SKIP=flake8 pre-commit run --files ggml/src/ggml-cuda/vendors/hip.h
• cmake -S . -B build
• cmake --build build -j 2

---

https://chatgpt.com/codex/tasks/task_b_688ef6d189fc83259cabdf198bb9e0f1