Fix compile error due to missing header (NVIDIA#11257)

The recently merged PR (rapidsai/cudf#11137) did not include the `<optional>` header which may cause compile error in some systems (in particular, CUDA 11.7 + gcc-11.2):
```
error: ‘std::optional’ has not been declared
error: ‘optional’ in namespace ‘std’ does not name a template type
```

This PR adds that missing header to fix the compile issue.

Authors:
  - Nghia Truong (https://github.com/ttnghia)

Approvers:
  - Gera Shegalov (https://github.com/gerashegalov)
  - David Wendt (https://github.com/davidwendt)
  - Mike Wilson (https://github.com/hyperbolic2346)

URL: rapidsai/cudf#11257

cpp/include/cudf/detail/null_mask.cuh

@@ -39,6 +39,7 @@
 
 #include <algorithm>
 #include <iterator>
+#include <optional>
 #include <vector>
 
 namespace cudf {

cpp/include/cudf/detail/reduction_functions.hpp

@@ -23,6 +23,8 @@
 
 #include <rmm/cuda_stream_view.hpp>
 
+#include <optional>
+
 namespace cudf {
 namespace reduction {
 /**

cpp/include/cudf/reduction.hpp

@@ -21,6 +21,8 @@
 
 #include <rmm/mr/device/per_device_resource.hpp>
 
+#include <optional>
+
 namespace cudf {
 /**
  * @addtogroup aggregation_reduction

cpp/src/reductions/simple_segmented.cuh

@@ -38,6 +38,7 @@
 #include <thrust/iterator/transform_iterator.h>
 #include <thrust/iterator/zip_iterator.h>
 
+#include <optional>
 #include <type_traits>
 
 namespace cudf {