Added missing prototype for the rebuild test function in cell.h

src/cell.h

@@ -390,4 +390,6 @@ void cell_activate_subcell_tasks(struct cell *ci, struct cell *cj,
                                  struct scheduler *s);
 int cell_can_recurse_in_pair_task(const struct cell *c);
 int cell_can_split_task(const struct cell *c);
+int cell_need_rebuild_for_pair(const struct cell *ci, const struct cell *cj);
+
 #endif /* SWIFT_CELL_H */