Fix MSVC error C2248 in FaustGPU

 c:\gitlab-runner\builds\0f08e4e7\0\faustgrp\faust\src\faust_linear_operator\cpu\faust_gpu_mod_double.hpp(206): error C2248: 'Faust::FaustGPU<double>::gp_funcs': cannot access private member declared in class 'Faust::FaustGPU<double>' [C:\Gitlab-Runner\builds\0f08e4e7\0\faustgrp\faust\build\wrapper\python\faust_python.vcxproj]
 c:\gitlab-runner\builds\0f08e4e7\0\faustgrp\faust\src\faust_linear_operator\cpu\faust_gpu_mod_double.hpp(208): error C2248: 'Faust::FaustGPU<double>::cpu_gpu_map': cannot access private member declared in class 'Faust::FaustGPU<double>' [C:\Gitlab-Runner\builds\0f08e4e7\0\faustgrp\faust\build\wrapper\python\faust_python.vcxproj]