printers.py (build_libstdcxx_dictionary): Fix std::_Fwd_list_iterator and std::_Fwd_list_const_iterator printers registration.

2018-03-09  François Dumont  <fdumont@gcc.gnu.org>

	* python/libstdcxx/v6/printers.py (build_libstdcxx_dictionary):
	Fix std::_Fwd_list_iterator and std::_Fwd_list_const_iterator printers
	registration.

From-SVN: r258380