Add missing header for GPU tests.

test/gpu_test_helper.h

diff --git a/test/gpu_test_helper.h b/test/gpu_test_helper.h
index 8955cbc..008e95c 100644
--- a/test/gpu_test_helper.h
+++ b/test/gpu_test_helper.h
@@ -3,6 +3,9 @@
 
 #include <Eigen/Core>
 
+// Allow gpu** macros for generic tests.
+#include <unsupported/Eigen/CXX11/src/Tensor/TensorGpuHipCudaDefines.h>
+
 // std::tuple cannot be used on device, and there is a bug in cuda < 9.2 that
 // doesn't allow std::tuple to compile for host code either. In these cases,
 // use our custom implementation.