make tm struct members public

This PR makes the members of `struct tm` public.

I see no reason why these fields should not be public fields. They are public in the other targets. I did a blame and it seems that they have been private since added a couple years ago.

https://github.com/rust-lang/libc/commit/53e0d387f8765e9faa1ea921450167bb7f0b31ff