Build warnings in `<cuda/std/barrier>`

[sleeepyjack]

Having build warnings when CMAKE_CUDA_ARCHITECTURES=75:

[ 98%] Linking CXX executable DYNAMIC_MAP_TEST
[ 98%] Built target DYNAMIC_MAP_TEST
/home/yunsongw/miniconda3/include/rapids/libcudacxx/cuda/std/barrier: In function ‘void cuda::__4::init(cuda::__4::barrier<cuda::std::__4::__detail::thread_scope_block>*, ptrdiff_t, cuda::std::__4::__empty_completion)’:
/home/yunsongw/miniconda3/include/rapids/libcudacxx/cuda/std/barrier:158:155: warning: unused parameter ‘__completion’ [-Wunused-parameter]
  158 |     friend void init(barrier * __b, std::ptrdiff_t __expected, std::__empty_completion __completion = std::__empty_completion()) {
      |                                                                                                                        ~~~~~~~~~~~                        ^                                                  
/home/yunsongw/miniconda3/include/rapids/libcudacxx/cuda/std/barrier: In function ‘void cuda::__4::init(cuda::__4::barrier<cuda::std::__4::__detail::thread_scope_block>*, ptrdiff_t, cuda::std::__4::__empty_completion)’:
/home/yunsongw/miniconda3/include/rapids/libcudacxx/cuda/std/barrier:158:155: warning: unused parameter ‘__completion’ [-Wunused-parameter]
  158 |     friend void init(barrier * __b, std::ptrdiff_t __expected, std::__empty_completion __completion = std::__empty_completion()) {
      |                                                                                                                        ~~~~~~~~~~~                        ^                                                  
/home/yunsongw/miniconda3/include/rapids/libcudacxx/cuda/std/barrier: In function ‘void cuda::__4::init(cuda::__4::barrier<cuda::std::__4::__detail::thread_scope_block>*, ptrdiff_t, cuda::std::__4::__empty_completion)’:
/home/yunsongw/miniconda3/include/rapids/libcudacxx/cuda/std/barrier:158:155: warning: unused parameter ‘__completion’ [-Wunused-parameter]
  158 |     friend void init(barrier * __b, std::ptrdiff_t __expected, std::__empty_completion __completion = std::__empty_completion()) {
      |                                                                                                                        ~~~~~~~~~~~                        ^                                                  

Worthing reporting to libcudacxx.

Otherwise, LGTM.

Originally posted by @PointKernel in https://github.com/NVIDIA/cuCollections/pull/201#pullrequestreview-1062428201