Add retry for building index

src/share/ob_ddl_task_executor.h

@@ -31,6 +31,7 @@ enum ObIDDLTaskType {
   DDL_TASK_SCHEDULE_BUILD_INDEX = 1,
   DDL_TASK_RS_BUILD_INDEX = 2,
   DDL_TASK_REFRESH_MEMORY_PERCENTAGE = 3,
+  DDL_TASK_RETRY_GHOST_INDEX = 4,
 };
 
 class ObIDDLTask : public common::ObDLinkBase<ObIDDLTask> {