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

diff --git a/Eigen/Core b/Eigen/Core
index bb8ad46..688361d 100644
--- a/Eigen/Core
+++ b/Eigen/Core
@@ -72,6 +72,7 @@
 #include <cmath>
 #include <cassert>
 #include <functional>
+#include <sstream>
 #ifndef EIGEN_NO_IO
   #include <iosfwd>
 #endif

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

diff --git a/Eigen/src/Core/arch/Default/Half.h b/Eigen/src/Core/arch/Default/Half.h
index 56782b3..9f4c1eb 100644
--- a/Eigen/src/Core/arch/Default/Half.h
+++ b/Eigen/src/Core/arch/Default/Half.h
@@ -42,6 +42,7 @@
 #define EIGEN_EXPLICIT_CAST(tgt_type) operator tgt_type()
 #endif
 
+#include <sstream>
 
 namespace Eigen {