Fix in6_addr alignment on Windows.

The alignment for in6_addr is 2 on Windows, but the original bindings set it to 4 on all platforms. This caused the Rust struct's alignment to differ from what libslirp expected on the C side, which meant that field reads on the C side were looking at potentially random or from its view, corrupted data. Here, we've changed the structure alignment to be correct based upon the platform detected at compile time.