Use thread fence in Actor.getSum()

tests/SimpleActorTest.cpp

@@ -5,6 +5,7 @@
 #include "Debug.hpp"
 #include "emper.hpp"
 
+#include <atomic>
 #include <mutex>
 
 class SumActor : public Actor<uint64_t> {
@@ -23,6 +24,7 @@ public:
 	}
 
 	uint64_t getSum() {
+		std::atomic_thread_fence(std::memory_order::memory_order_acquire);
 		return sum;
 	}
 };