Add a missing #include to tramp3d-v4.cpp

This was failing to build because of a libcxx change: https://reviews.llvm.org/D71214
diff --git a/MultiSource/Benchmarks/tramp3d-v4/tramp3d-v4.cpp b/MultiSource/Benchmarks/tramp3d-v4/tramp3d-v4.cpp
index 2d053dc..8f9afb8 100644
--- a/MultiSource/Benchmarks/tramp3d-v4/tramp3d-v4.cpp
+++ b/MultiSource/Benchmarks/tramp3d-v4/tramp3d-v4.cpp
@@ -8,6 +8,7 @@
 #include <limits>
 #include <math.h>
 #include <cstring>
+#include <sstream>
 
 namespace Smarts {
 template<class T>