Fix for the HIP build+test errors.

The errors were introduced by this commit :

After the above mentioned commit, some of the tests started failing with the following error


```
Built target cxx11_tensor_reduction
Building HIPCC object unsupported/test/CMakeFiles/cxx11_tensor_reduction_gpu_5.dir/cxx11_tensor_reduction_gpu_5_generated_cxx11_tensor_reduction_gpu.cu.o
In file included from /home/rocm-user/eigen/unsupported/test/cxx11_tensor_reduction_gpu.cu:16:
In file included from /home/rocm-user/eigen/unsupported/Eigen/CXX11/Tensor:117:
/home/rocm-user/eigen/unsupported/Eigen/CXX11/src/Tensor/TensorBlockV2.h:155:5: error: the field type is not amp-compatible
    DestinationBufferKind m_kind;
    ^
/home/rocm-user/eigen/unsupported/Eigen/CXX11/src/Tensor/TensorBlockV2.h:211:3: error: the field type is not amp-compatible
  DestinationBuffer m_destination;
  ^
```


For some reason HIPCC does not like device code to contain enum types which do not have the base-type explicitly declared. The fix is trivial, explicitly state "int" as the basetype

unsupported/Eigen/CXX11/src/Tensor/TensorBlockV2.h

diff --git a/unsupported/Eigen/CXX11/src/Tensor/TensorBlockV2.h b/unsupported/Eigen/CXX11/src/Tensor/TensorBlockV2.h
index c85c4c6..48417c1 100644
--- a/unsupported/Eigen/CXX11/src/Tensor/TensorBlockV2.h
+++ b/unsupported/Eigen/CXX11/src/Tensor/TensorBlockV2.h
@@ -80,7 +80,14 @@
   // evaluated expression scalar type.
   class DestinationBuffer {
    public:
-    enum DestinationBufferKind {
+    enum DestinationBufferKind : int {
+      // The above explicit specification of "int" as the enum basetype is needed
+      // to get around a HIPCC link error ("the field type is not amp-compatible")
+      // which is issued for class members with the enum type.
+      // TODO(rocm):
+      // remove the "int" basetype once HIPCC has been fixed to not error out
+      // in the above scenario.
+
       // Destination buffer is not defined (`m_data` == nullptr).
       kEmpty,