fix import in cfail test

Author: Jorge Aparicio

src/test/compile-fail/issue-17728.rs

@@ -8,7 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-use std::fmt::{Show, Formatter, Error};
+use std::fmt::{Debug, Formatter, Error};
 use std::collections::HashMap;
 
 trait HasInventory {