Avoid size_t to int conversion warning in visit_each_test.cpp

test/visit_each_test.cpp

@@ -45,7 +45,7 @@ struct V
 
     void operator()( std::string const & w )
     {
-        s_ = s_ * 10 + w.size();
+        s_ = s_ * 10 + static_cast<int>( w.size() );
     }
 };