Fix unused parameter warning.

asio/include/asio/detail/impl/signal_set_service.ipp

diff --git a/asio/include/asio/detail/impl/signal_set_service.ipp b/asio/include/asio/detail/impl/signal_set_service.ipp
index 1399514..cec0408 100644
--- a/asio/include/asio/detail/impl/signal_set_service.ipp
+++ b/asio/include/asio/detail/impl/signal_set_service.ipp
@@ -380,7 +380,7 @@
   {
   }
 
-  static bool do_perform(reactor_op* base)
+  static bool do_perform(reactor_op*)
   {
     signal_state* state = get_signal_state();