I’m not really an expert (though I play one on TV – jk 
). But searching around the OpenThread repo, it looks like that ProcessFactoryReset method just calls void otInstanceFactoryReset(otInstance *aInstance), which is defined in instance_api.cpp.
I’m assuming that the instance_api.cpp file is part of the build, so I’m guessing that just adding that function’s forward declaration to your extern "C" block will let you use it? But I’m not really positive. Give it a try! 