Fix compilation with boost 1.66.0

Change-Id: Ibf2883a974c834a5281c39a551e6d467d4454501
diff --git a/src/util/scheduler.hpp b/src/util/scheduler.hpp
index 2139c76..6d04bd1 100644
--- a/src/util/scheduler.hpp
+++ b/src/util/scheduler.hpp
@@ -23,8 +23,8 @@
 #define NDN_UTIL_SCHEDULER_HPP
 
 #include "time.hpp"
+#include "../net/asio-fwd.hpp"
 
-#include <boost/asio/io_service.hpp>
 #include <set>
 
 namespace ndn {