Include <thread> for std::this_thread::yield().

Eigen/Core

diff --git a/Eigen/Core b/Eigen/Core
index e452e73..29dda39 100644
--- a/Eigen/Core
+++ b/Eigen/Core
@@ -97,6 +97,11 @@
 // for std::is_nothrow_move_assignable
 #include <type_traits>
 
+// for std::this_thread::yield().
+#if !defined(EIGEN_USE_BLAS) && (defined(EIGEN_HAS_OPENMP) || defined(EIGEN_GEMM_THREADPOOL))
+#include <thread>
+#endif
+
 // for outputting debug info
 #ifdef EIGEN_DEBUG_ASSIGN
 #include <iostream>
@@ -117,8 +122,8 @@
 #include <CL/sycl.hpp>
 #include <map>
 #include <memory>
-#include <utility>
 #include <thread>
+#include <utility>
 #ifndef EIGEN_SYCL_LOCAL_THREAD_DIM0
 #define EIGEN_SYCL_LOCAL_THREAD_DIM0 16
 #endif