[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 094c188..e4ef3b6 100644
--- a/core/scheduler.cpp
+++ b/core/scheduler.cpp
@@ -36,8 +36,9 @@
 Scheduler&
 getGlobalScheduler()
 {
+  static ndn::DummyIoService io;
   if (g_scheduler.get() == nullptr) {
-    g_scheduler.reset(new Scheduler(getGlobalIoService()));
+    g_scheduler.reset(new Scheduler(io));
   }
 
   return *g_scheduler;