Fix for clang compilation

sql/sql_type_json.h

@@ -81,7 +81,7 @@ class Type_handler_general_purpose_string_to_json:
   }
 
   bool Item_append_extended_type_info(Send_field_extended_metadata *to,
-                                      const Item *item) const
+                                      const Item *item) const override
   {
     return set_format_name(to); // Send "format=json" in the protocol
   }