Support /MP parallel compilation with CMake MSVC generator

By default, building with CMake and the Visual Studio generators on Windows yields a solution that isn't built with multithreading.

I think 5 lines of CMake are actually sufficient as I've done here: [CMakeLists.txt#L508](https://github.com/TomClabault/HIPRT/blob/f87fb182b0323c13947bdf0a5c16ddf413be2ccd/CMakeLists.txt#L508)