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.

peglib.h

@@ -1104,6 +1104,8 @@ struct Ope::Visitor
 
 struct AssignIDToDefinition : public Ope::Visitor
 {
+    using Ope::Visitor::visit;
+
     void visit(Sequence& ope) override {
         for (auto op: ope.opes_) {
             op->accept(*this);
@@ -1133,6 +1135,8 @@ struct IsToken : public Ope::Visitor
 {
     IsToken() : has_token_boundary(false), has_rule(false) {}
 
+    using Ope::Visitor::visit;
+
     void visit(Sequence& ope) override {
         for (auto op: ope.opes_) {
             op->accept(*this);
@@ -1661,6 +1665,8 @@ private:
         DetectLeftRecursion(const std::string& name)
             : s_(nullptr), name_(name), done_(false) {}
 
+        using Ope::Visitor::visit;
+
         void visit(Sequence& ope) override {
             for (auto op: ope.opes_) {
                 op->accept(*this);