document sstream inclusion

test/source/greeter.cpp

@@ -1,7 +1,10 @@
 #include <doctest/doctest.h>
 #include <greeter.h>
 
-#include <sstream>
+#if defined(_WIN32) || defined(WIN32)
+// apparently this is required to compile in MSVC++
+#  include <sstream>
+#endif
 
 TEST_CASE("Greeter") {
   using namespace greeter;