Add missing 'explicit's on single-argument constructors.

Change-Id: I1494df6e74ec16238971fb4346ba184eb61c37ab
diff --git a/src/class_linker.h b/src/class_linker.h
index 2a3e74d..3a44310 100644
--- a/src/class_linker.h
+++ b/src/class_linker.h
@@ -230,7 +230,7 @@
       ObjectArray<Method>* methods, ObjectArray<ObjectArray<Class> >* throws);
 
  private:
-  ClassLinker(InternTable*);
+  explicit ClassLinker(InternTable*);
 
   // Initialize class linker by bootstraping from dex files
   void Init(const std::string& boot_class_path);