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 _))?;
       |                                  ^^^^^^^^^^^^^^^^^

src/sys/unix/size.rs

@@ -14,7 +14,7 @@ struct TermSize {
 pub fn terminal_size() -> io::Result<(u16, u16)> {
     unsafe {
         let mut size: TermSize = mem::zeroed();
-        cvt(ioctl(STDOUT_FILENO, TIOCGWINSZ, &mut size as *mut _))?;
+        cvt(ioctl(STDOUT_FILENO, TIOCGWINSZ.into(), &mut size as *mut _))?;
         Ok((size.col as u16, size.row as u16))
     }
 }