Move non-pub import into correct place

We have a non-public import that has snuck in below then public ones.
This isn't caught by the formatter because we explicitly skip on the
public exports.

Author: tcharding

primitives/src/locktime/relative.rs

@@ -10,6 +10,7 @@ use core::{convert, fmt};
 #[cfg(feature = "arbitrary")]
 use arbitrary::{Arbitrary, Unstructured};
 use internals::write_err;
+use units::{BlockHeight, BlockMtp};
 
 use crate::Sequence;
 
@@ -19,7 +20,6 @@ use crate::relative;
 #[rustfmt::skip]                // Keep public re-exports separate.
 #[doc(inline)]
 pub use units::locktime::relative::{NumberOfBlocks, NumberOf512Seconds, TimeOverflowError, InvalidHeightError, InvalidTimeError};
-use units::{BlockHeight, BlockMtp};
 
 #[deprecated(since = "TBD", note = "use `NumberOfBlocks` instead")]
 #[doc(hidden)]