fix inconsistency between env_hdfs and env

hdfs/env_hdfs.h

@@ -101,7 +101,7 @@ class HdfsEnv : public Env {
                            std::shared_ptr<Logger>* result);
 
   virtual void Schedule(void (*function)(void* arg), void* arg,
-                        Priority pri = LOW, void* tag = nullptr) {
+                        Priority pri = LOW, void* tag = nullptr, void (*unschedFunction)(void* arg) = 0) {
     posixEnv->Schedule(function, arg, pri, tag);
   }