Fix compiler warnings about incomplete override functions
Fix compiler warnings about incomplete override functions

The warning on VS2013 was:

warning C4266: 'void peg::Ope::Visitor::visit(peg::Whitespace &)' : no
  override available for virtual member function from base
  'peg::Ope::Visitor'; function is hidden

I'm surprised that this warning doesn't happen on gcc/clang though.

yhirose/cpp-peglib