Auto merge of rust-lang#28054 - zaeleus:ios-imports, r=alexcrichton

This fixes building for ios targets caused by 7925c79.

Author: bors

src/libstd/sys/unix/backtrace/tracing/backtrace_fn.rs

@@ -20,18 +20,15 @@
 
 use io;
 use io::prelude::*;
-use iter::Iterator;
 use libc;
 use mem;
 use result::Result::Ok;
 use sync::StaticMutex;
 
-use super::printer::print;
+use super::super::printing::print;
 
 #[inline(never)]
 pub fn write(w: &mut Write) -> io::Result<()> {
-    use mem;
-
     extern {
         fn backtrace(buf: *mut *mut libc::c_void,
                      sz: libc::c_int) -> libc::c_int;