Cleanup iostream includes

Change-Id: I16dd945998463975b140c7a010d64a112f95d8ba
diff --git a/src/interest.cpp b/src/interest.cpp
index 3f8352b..26b564e 100644
--- a/src/interest.cpp
+++ b/src/interest.cpp
@@ -24,6 +24,7 @@
 #include "data.hpp"
 
 #include <cstring>
+#include <sstream>
 
 namespace ndn {
 
@@ -169,6 +170,14 @@
   }
 }
 
+std::string
+Interest::toUri() const
+{
+  std::ostringstream os;
+  os << *this;
+  return os.str();
+}
+
 // ---- matching ----
 
 bool