Give Rule a virtual destructor

Not needed at the moment because rule pointers are are always wrapped
in shared_ptrs. Still, don't want to forget this if I stopped using shared_ptrs
at some point.

src/compiler/rules/rule.cc

@@ -21,5 +21,7 @@ namespace tree_sitter {
                 stream << string("#<null-rule>");
             return stream;
         }
+
+        Rule::~Rule() {}
     }
 }

src/compiler/rules/rule.h

@@ -19,6 +19,7 @@ namespace tree_sitter {
             virtual rule_ptr copy() const = 0;
             virtual std::string to_string() const = 0;
             virtual void accept(Visitor *visitor) const = 0;
+            virtual ~Rule();
         };
 
         std::ostream& operator<<(std::ostream& stream, const Rule &rule);