Fix compiler error with cuda-clang

segment_reduction_ops.h requires cuda_kernel_helper.h to be
included in clang because it uses some of the helpers directly in the
header (e.g. CudaAtomicMax). It works with nvcc, because the usage is
in a template context and nvcc checks that function is available only
at template instantiation.
However, clang does more strict erorr-checking for functions found
during template instantiation and requires them to be found either by
ADL or at the point of template declaration.