Fix Mroonga compilation

Fixed warnings: -Woverloaded-virtual, -Winconsistent-missing-override

Caused by c9cba597

sql/handler.h

@@ -3688,7 +3688,7 @@ class handler :public Sql_alloc
     DBUG_PRINT("info",("auto_increment: next value %lu", (ulong)id));
     next_insert_id= id;
   }
-  void restore_auto_increment(ulonglong prev_insert_id)
+  virtual void restore_auto_increment(ulonglong prev_insert_id)
   {
     /*
       Insertion of a row failed, re-use the lastly generated auto_increment

storage/mroonga/ha_mroonga.hpp

@@ -571,7 +571,7 @@ public:
   void set_next_insert_id(ulonglong id);
   void get_auto_increment(ulonglong offset, ulonglong increment, ulonglong nb_desired_values,
                           ulonglong *first_value, ulonglong *nb_reserved_values) mrn_override;
-  void restore_auto_increment(ulonglong prev_insert_id);
+  void restore_auto_increment(ulonglong prev_insert_id) mrn_override;
   void release_auto_increment() mrn_override;
   int check_for_upgrade(HA_CHECK_OPT *check_opt) mrn_override;
 #ifdef MRN_HANDLER_HAVE_RESET_AUTO_INCREMENT