Fix duplicate variable warning.

[SVN r48415]

include/boost/asio/detail/win_iocp_io_service.hpp

@@ -531,7 +531,7 @@ private:
           // Wake up next thread that is blocked on GetQueuedCompletionStatus.
           if (!::PostQueuedCompletionStatus(iocp_.handle, 0, 0, 0))
           {
-            DWORD last_error = ::GetLastError();
+            last_error = ::GetLastError();
             ec = boost::system::error_code(last_error,
                 boost::asio::error::get_system_category());
             return 0;