build: Add missing include

Change-Id: Ie02d8c3c1422f0652659a3674a0e3f06287e8a6d
diff --git a/nsync/sync-socket.cc b/nsync/sync-socket.cc
index e3fbcf1..834dbee 100644
--- a/nsync/sync-socket.cc
+++ b/nsync/sync-socket.cc
@@ -20,6 +20,7 @@
 
 #include "sync-socket.h"
 #include "sync-logging.h"
+#include <boost/lexical_cast.hpp>
 
 using namespace std;
 using namespace ndn;
@@ -45,7 +46,8 @@
                  face,
                  bind(&SyncSocket::passCallback, this, _1),
                  rmCallback)
-{}
+{
+}
 
 SyncSocket::~SyncSocket()
 {