Cleanup iostream includes

Change-Id: I16dd945998463975b140c7a010d64a112f95d8ba
diff --git a/src/exclude.cpp b/src/exclude.cpp
index 88e51db..900d933 100644
--- a/src/exclude.cpp
+++ b/src/exclude.cpp
@@ -25,6 +25,7 @@
 #include "encoding/block-helpers.hpp"
 
 #include <boost/range/adaptor/reversed.hpp>
+#include <sstream>
 
 namespace ndn {