Don't provide io_service typedef when ASIO_NO_DEPRECATED is defined.

asio/include/asio/io_service.hpp

diff --git a/asio/include/asio/io_service.hpp b/asio/include/asio/io_service.hpp
index 4eb7cc5..3cb7448 100644
--- a/asio/include/asio/io_service.hpp
+++ b/asio/include/asio/io_service.hpp
@@ -21,8 +21,10 @@
 
 namespace asio {
 
+#if !defined(ASIO_NO_DEPRECATED)
 /// Typedef for backwards compatibility.
 typedef io_context io_service;
+#endif // !defined(ASIO_NO_DEPRECATED)
 
 } // namespace asio