Add missing import

Author: jonas-schievink

src/librustc_typeck/coherence/inherent_impls_overlap.rs

@@ -1,6 +1,6 @@
 use crate::namespace::Namespace;
 use rustc::traits::{self, IntercrateMode, SkipLeakCheck};
-use rustc::ty::TyCtxt;
+use rustc::ty::{AssocItem, TyCtxt};
 use rustc_errors::struct_span_err;
 use rustc_hir as hir;
 use rustc_hir::def_id::{CrateNum, DefId, LOCAL_CRATE};