Fix sass on apple

[tom-un]

When using go-libsass on a Mac on OSX 14.1.1, it would fail with "File to import not found or unreadable: " errors.

The issue is in libsass in the read_file function in file.cpp. On non-_WIN32 OS's it uses std::ifstream to read the file. On OSX it always fails with strerror(errno) saying "Failure: too many open files".

On my system, ulimit and ulimit -n return the default "unlimited" / 256. So I tried raising it in /Library/LaunchDaemons/limit.maxfiles.plist but it still didn't help.

I tried reverting the most recent change in file.cpp which used fopen instead of std::ifstream. It still failed.

So, I tried using the Darwin open/read and it works!