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

Having build warnings when `CMAKE_CUDA_ARCHITECTURES=75`:
```bash
[ 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*, 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*, 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*, 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_