fix missing header include in test

This omission was spotted when compiling with GCC-5 on Ubuntu/wily

tests/library/test/test-helper-demangling-test.cpp

@@ -25,6 +25,7 @@
 #include "lib/test/test-helper.hpp"
 
 #include <string>
+#include <iostream>
 
 using std::string;
 using std::cout;