der: fixup multiple-bounds-declaration warning

[baloo]

warning: bound is defined in more than one place
  --> der/src/ord.rs:56:31
   |
56 | pub(crate) fn iter_cmp<'a, I, T: 'a>(a: I, b: I) -> Result<Ordering>
   |                               ^
...
59 |     T: DerOrd,
   |     ^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#multiple_bound_locations
   = note: `#[warn(clippy::multiple_bound_locations)]` on by default