Add paths.h on macos

I saw sys/paths.h was added recently (https://github.com/ziglang/zig/pull/15951). I would like to have just paths.h! FWIW, I'm slowly converting external library dependencies of Acton https://github.com/actonlang/acton/ to be compiled using zig cc rather than the system compiler, so slowly noticing what I'm missing.