[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.cpp b/core/scheduler.cpp
index d0b0f2b..49ec6e6 100644
--- a/core/scheduler.cpp
+++ b/core/scheduler.cpp
@@ -37,7 +37,7 @@
 getGlobalScheduler()
 {
   if (g_scheduler.get() == nullptr) {
-    g_scheduler.reset(new Scheduler(getGlobalIoService()));
+    g_scheduler.reset(new Scheduler(*static_cast<boost::asio::io_service*>(nullptr)));
   }
 
   return *g_scheduler;