fix python build error when opencl headers not present
fix python build error when opencl headers not present

MiniLight/DeepCL,MiniLight/DeepCL,MiniLight/DeepCL,hughperkins/DeepCL,MiniLight/DeepCL,hughperkins/DeepCL,hughperkins/DeepCL,hughperkins/DeepCL,hughperkins/DeepCL,MiniLight/DeepCL