Mark the flags `volatile`, just to be safe. This is not a problem now, but I...

Mark the flags `volatile`, just to be safe. This is not a problem now, but I would hate it to become one later.

src/cell.h

@@ -269,7 +269,7 @@ struct cell {
   struct cell *super;
 
   /*! Cell flags bit-mask. */
-  uint32_t flags;
+  volatile uint32_t flags;
 
   /*! Hydro variables */
   struct {