Fix 'unused import' warning.

Fixes rust-lang#6630

Author: steveklabnik

src/libcore/vec.rs

@@ -22,7 +22,6 @@ use old_iter;
 use iterator::Iterator;
 use kinds::Copy;
 use libc;
-use old_iter::{BaseIter, CopyableIter};
 use option::{None, Option, Some};
 use ptr::to_unsafe_ptr;
 use ptr;
@@ -33,6 +32,9 @@ use unstable::intrinsics;
 use vec;
 use util;
 
+#[cfg(stage0)]
+use old_iter::{BaseIter, CopyableIter};
+
 #[cfg(not(test))] use cmp::Equiv;
 
 pub mod rustrt {