Add `sys/timex.h` on MacOS

sys/timex.h is part of libc, it is used by the NTP daemon and similar to get and set clock / time information. zig includes it on Linux but it is missing on MacOS! Please add :) Not sure if there is any more information I should add, feel free to ping me :) Also see https://github.com/ziglang/zig/issues/14840 which I originally opened for this issue.