[rdf] fix windows warning about redefined virtual functions in lexertk

(some) original messages:

lexertk.hpp(1017): warning C4266: 'int
lexertk::token_inserter::insert(const lexertk::token &,const
lexertk::token &,const lexertk::token &,const lexertk::token &,const
lexertk::token &,lexertk::token &)': no override available for virtual
member function from base 'lexertk::token_inserter'; function is hidden 

lexertk.hpp(1317): warning C4266: 'bool lexertk::token_scanner::operator
()(const lexertk::token &,const lexertk::token &,const lexertk::token
&,const lexertk::token &)': no override available for virtual member
function from base 'lexertk::token_scanner'; function is hidden