[SYCL] Cast some unused variable as void in bfloat16.hpp

[yubingex007-a11y]

No description provided.

[yubingex007-a11y]

i am trying to fix https://github.com/intel/llvm/runs/6310070794?check_suite_focus=true
if #5954 need more time, i guess we can merge this pr, right?

[steffenlarsen]

Failures unrelated:

SYCL / Linux / HIP AMDGPU LLVM Test Suite (pull_request)

• SYCL :: Reduction/reduction_span.cpp (fixed in [SYCL] Expand reduction_span XFAIL to all hip backend configs llvm-test-suite#1020)

Jenkins/Precommit

• SYCL :: Basic/sampler/sampler.cpp (Known failure)