clock: change num primitive type to usize in nom call

This fix build error on 32bit platform
where ToUsize is not implemented for u64.

Bug reported by @jonassmedegaard

src/vobsub/mpeg2/clock.rs

@@ -51,10 +51,10 @@ impl fmt::Display for Clock {
 
 /// Parse a 33-bit `Clock` value with 3 marker bits, consuming 36 bits.
 pub fn clock(i: (&[u8], usize)) -> IResult<(&[u8], usize), Clock> {
- let marker = tag(0b1, 1u8);
- let hi_p = take(3u64);
- let mid_p = take(15u64);
- let lo_p = take(15u64);
+ let marker = tag(0b1, 1usize);
+ let hi_p = take(3usize);
+ let mid_p = take(15usize);
+ let lo_p = take(15usize);
 
  let (input, (hi, _, mid, _, lo, _)): ((&[u8], usize), (u64, _, u64, _, u64, _)) =
  (hi_p, &marker, mid_p, &marker, lo_p, &marker).parse(i)?;