face+fw: store and return PitToken from downstream

refs #4532

Change-Id: I97b95232a242186601459622f17983d1bef806d6
diff --git a/core/common.hpp b/core/common.hpp
index efc9856..4280dc7 100644
--- a/core/common.hpp
+++ b/core/common.hpp
@@ -124,6 +124,7 @@
 namespace signal = ndn::util::signal;
 namespace time = ndn::time;
 using namespace ndn::time_literals;
+using ndn::operator""_block;
 
 } // namespace nfd