fix: add missed symbol (fastify#4608)

Author: metcoder95

lib/symbols.js

@@ -30,6 +30,7 @@ const keys = {
   kRequestValidateFns: Symbol('fastify.request.cache.validateFns'),
   kRequestPayloadStream: Symbol('fastify.RequestPayloadStream'),
   kRequestAcceptVersion: Symbol('fastify.RequestAcceptVersion'),
+  kRequestValidateWeakMap: Symbol('fastify.request.cache.validators'),
   // 404
   kFourOhFour: Symbol('fastify.404'),
   kCanSetNotFoundHandler: Symbol('fastify.canSetNotFoundHandler'),