Define a custom attribute to lv_flush_ready function

lv_core/lv_vdb.c

@@ -152,7 +152,7 @@ void lv_vdb_set_adr(void * buf1, void * buf2)
 /**
  * Call in the display driver's  'disp_flush' function when the flushing is finished
  */
-void lv_flush_ready(void)
+LV_ATTRIBUTE_FLUSH_READY void lv_flush_ready(void)
 {
 #if LV_VDB_DOUBLE == 0
     vdb_state = LV_VDB_STATE_ACTIVE;