Give rules::Visitor a virtual destructor

src/compiler/rules/visitor.cc

@@ -12,8 +12,10 @@
 
 namespace tree_sitter {
     using std::vector;
-    
+
     namespace rules {
+        Visitor::~Visitor() {}
+
         rule_ptr IdentityRuleFn::default_apply(const Rule *rule) {
             return rule->copy();
         }

src/compiler/rules/visitor.h

@@ -28,6 +28,7 @@ namespace tree_sitter {
             virtual void visit(const String *rule) = 0;
             virtual void visit(const Symbol *rule) = 0;
             virtual void visit(const ISymbol *rule) = 0;
+            virtual ~Visitor();
         };
 
         template<typename T>