Cleanup iostream includes

Change-Id: I16dd945998463975b140c7a010d64a112f95d8ba
diff --git a/tests/unit-tests/encoding/tlv.t.cpp b/tests/unit-tests/encoding/tlv.t.cpp
index 8138e8a..52b46ad 100644
--- a/tests/unit-tests/encoding/tlv.t.cpp
+++ b/tests/unit-tests/encoding/tlv.t.cpp
@@ -26,6 +26,7 @@
 #include <array>
 #include <deque>
 #include <list>
+#include <sstream>
 #include <boost/concept_archetype.hpp>
 #include <boost/iostreams/stream.hpp>
 #include <boost/iostreams/device/array.hpp>