Add missing local declarations for lint

Author: falvaradorodriguez

apisix/plugins/limit-req/util.lua

@@ -16,6 +16,8 @@
 --
 local ngx_now           = ngx.now
 local tonumber          = tonumber
+local tostring          = tostring
+local type              = type
 local core              = require("apisix.core")
 local resty_string      = require("resty.string")