crimson/common/operation: fix decltype usage

```
ceph/src/crimson/common/operation.cc
In file included from /home/matan/ceph/src/crimson/common/operation.cc:4:
/home/matan/ceph/src/crimson/common/operation.h:312:31: error: no member named 'events' in 'AggregateBlockingEvent<T>'
  312 |       typename decltype(event.events)::iterator iter;
      |                         ~~~~~ ^
1 error generated.
```

Signed-off-by: Matan Breizman <[email protected]>

Author: Matan-B

src/crimson/common/operation.h

@@ -309,7 +309,7 @@ struct AggregateBlockingEvent {
 
     class TriggerContainer final : public TriggerI::TriggerContainerI {
       AggregateBlockingEvent& event;
-      typename decltype(event.events)::iterator iter;
+      typename std::list<T>::iterator iter;
       typename T::template Trigger<OpT> trigger;
 
       typename T::TriggerI &get_trigger() final {