Fix for perf/platform.hpp not being cleaned up by distclean

`platform.hpp` was being copied to `/perf` by the msvc projects.  This was not cleaned up by `make distclean` when preparing for another platform build.  The file was already being copied to `/src` as part of the libzmq core project build, so revised the projects to copy to `/src` as well, and changed `#include` statements in affected `/perf` modules to reference the new location.