Include <sstream> explicitly, and don't rely on the implicit include via <complex>.

This implicit dependency does no longer exist in a recent llbm release (sha 78be61871704).

Eigen/Core

@@ -279,7 +279,10 @@
 #include <cmath>
 #include <cassert>
 #include <functional>
-#include <iosfwd>
+#include <sstream>
+#ifndef EIGEN_NO_IO
+  #include <iosfwd>
+#endif
 #include <cstring>
 #include <string>
 #include <limits>

Eigen/src/Core/arch/CUDA/Half.h

@@ -42,6 +42,7 @@
 #define EIGEN_EXPLICIT_CAST(tgt_type) operator tgt_type()
 #endif
 
+#include <sstream>
 
 namespace Eigen {