fix: avoid nauty unistd.h include under MSVC

[wshlavacek]

src/nauty24/nauty.h currently hardcodes HAVE_UNISTD_H 1, so MSVC builds still reach #include <unistd.h> even though that header is not available there.

This patch applies the smallest targeted fix by setting HAVE_UNISTD_H to 0 under _MSC_VER only. Non-MSVC builds, including existing MinGW-style Windows builds, keep the current behavior.

Verification:

• reproduced the pre-patch include path with a small compile probe
• confirmed a simulated MSVC build no longer reaches unistd.h
• confirmed a simulated MinGW-style build still follows the existing include path
• rebuilt NFsim locally with CMake and confirmed NFsim -h still runs

I could not run a real Windows/MSVC build locally.