[ndnSIM] core: Partial emulation of boost::asio::io_service for NS-3

This commit assumes that ndn::Scheduler is redirected to ns3::Simulator
routines inside the ndn-cxx library.

Change-Id: Ib987f09f76a5d2103365312261fc5a2eba7024d2
diff --git a/core/scheduler.hpp b/core/scheduler.hpp
index b846c64..6e87199 100644
--- a/core/scheduler.hpp
+++ b/core/scheduler.hpp
@@ -49,6 +49,9 @@
 void
 cancel(const EventId& eventId);
 
+Scheduler&
+getGlobalScheduler();
+
 /** \brief cancels an event automatically upon destruction
  */
 class ScopedEventId : noncopyable