do not alias virtual_ptr in global namespace, 'aliases' namespace

examples/default_error_handler.cpp

@@ -10,6 +10,8 @@
 #include <boost/openmethod.hpp>
 #include <boost/openmethod/compiler.hpp>
 
+using boost::openmethod::virtual_ptr;
+
 struct Animal {
     virtual ~Animal() = default;
 };