Remove use of deprecated code

Change-Id: I197471c48c54c5671bb8f641c4332b3ac7893824
Refs: #3988
diff --git a/nsync/sync-logic.h b/nsync/sync-logic.h
index 33578fb..6ac330b 100644
--- a/nsync/sync-logic.h
+++ b/nsync/sync-logic.h
@@ -127,7 +127,7 @@
   onSyncRegisterFailed(const ndn::Name& prefix, const std::string& msg);
 
   void
-  onSyncData(const ndn::Interest& interest, ndn::Data& data);
+  onSyncData(const ndn::Interest& interest, const ndn::Data& data);
 
   void
   onSyncTimeout(const ndn::Interest& interest);