EnumIface: suppress deprecation warning (#540)

The EnumIterator inherits from std::iterator, which has
been deprecated in c++17. There is a fix in gz-common6
but it breaks API, so it can't be merged to Garden. So
just suppress the deprecation warning in Garden.

Signed-off-by: Steve Peters <scpeters@openrobotics.org>

include/gz/common/EnumIface.hh

@@ -141,6 +141,7 @@ namespace gz
     ///   std::cout << "Type++ Name[" << myTypeIface.Str(*i) << "]\n";
     /// }
     /// \verbatim
+    GZ_UTILS_WARN_IGNORE__DEPRECATED_DECLARATION
     template<typename Enum>
     class EnumIterator
     : std::iterator<std::bidirectional_iterator_tag, Enum>
@@ -218,6 +219,7 @@ namespace gz
       /// member value ever used.
       private: Enum c;
     };
+    GZ_UTILS_WARN_RESUME__DEPRECATED_DECLARATION
 
     /// \brief Equality operator
     /// \param[in] _e1 First iterator