Fix TIOCGWINSZ type mismatch on DragonFly

Below is the error message I got before this patch:

    error[E0308]: mismatched types
      --> src/sys/unix/size.rs:17:34
       |
    17 |         cvt(ioctl(STDOUT_FILENO, TIOCGWINSZ, &mut size as *mut _))?;
       |                                  ^^^^^^^^^^ expected u64, found u32
    help: you can cast an `u32` to `u64`, which will zero-extend the source value
       |
    17 |         cvt(ioctl(STDOUT_FILENO, TIOCGWINSZ.into(), &mut size as *mut _))?;
       |                                  ^^^^^^^^^^^^^^^^^