ensure Worker copy fns private and undefined

Author: kevinushey

inst/include/RcppParallel/Common.h

@@ -21,6 +21,7 @@ struct Worker
    virtual void operator()(std::size_t begin, std::size_t end) = 0;   
 
    // disable copying and assignment
+private:
    Worker(const Worker&);
    void operator=(const Worker&);
 };