reducecpp sample not passing correct -cl-std version

[bashbaug]

The "reducecpp" sample tries to build an OpenCL kernel that uses the work-group reduction functions, but does not pass a -cl-std version for this kernel. Since these functions require OpenCL C 2.0 or OpenCL C 3.1 or newer with the __opencl_c_work_group_collective_functions feature, some compilers will diagnose an error for this kernel.

This sample should pass a proper -cl-std build option for this kernel instead, similar to what is done for the non-C++ "reduce" sample.