data+interest: Fixing operator<<

Data packet was missing declaration of operator<< in header file.
Interest class has incorrect output of InterestLifetime parameter
(should be just number of milliseconds, without word "milliseconds"
present).

Change-Id: If2249ed033ab4b47640dfa4ea08253fdb84373d6
Refs: #1681
diff --git a/src/data.hpp b/src/data.hpp
index 70d1a72..1480103 100644
--- a/src/data.hpp
+++ b/src/data.hpp
@@ -321,6 +321,8 @@
   friend class nfd::LocalControlHeader;
 };
 
+std::ostream&
+operator<<(std::ostream& os, const Data& data);
 
 inline bool
 Data::hasWire() const