Fix clang 6.0 warnings in tests: unused members, suspicious return types

example/doc_how_to_use.cpp

@@ -23,6 +23,9 @@ class MyClass : public list_base_hook<>
    list_member_hook<> member_hook_;
 
    MyClass(int i) :  int_(i)  {}
+   //<-
+   int get_int() const { return int_; }
+   //->
 };
 
 //Define a list that will store MyClass using the base hook