Add needed virtual destructor to the (map) Worker class.

This is needed because MapUpdater::WorkerThread() can delete objects
of derived classes, like MapUpdateWorker, via a pointer to base class,
and without a virtual destructor this isn't calling the correct
destructor!

Closes #499

src/game/Maps/MapWorkers.h

@@ -30,6 +30,7 @@ class Worker
 {
     public:
         Worker(MapUpdater& updater) : m_updater(updater) {}
+        virtual ~Worker() = default;
         virtual void execute() {};
 
     protected:
@@ -107,4 +108,4 @@ class ObjectUpdateWorker : public Worker
         uint32 m_diff;
 };
 
-#endif //_MAP_WORKERS_H_INCLUDED
+#endif //_MAP_WORKERS_H_INCLUDED