Fixed graceful shutdown.

src/workers/Hashrate.h

@@ -37,6 +37,7 @@ public:
     double calc(size_t threadId, size_t ms) const;
     void add(size_t threadId, uint64_t count, uint64_t timestamp);
     void print();
+    void stop();
     void updateHighest();
 
     inline double highest() const { return m_highest; }