Added Benchmark class.

src/backend/cpu/CpuBackend.h

@@ -70,6 +70,11 @@ protected:
     void handleRequest(IApiRequest &request) override;
 #   endif
 
+#   ifdef XMRIG_FEATURE_BENCHMARK
+    Benchmark *benchmark() const override;
+    void printBenchProgress() const override;
+#   endif
+
 private:
     CpuBackendPrivate *d_ptr;
 };