include: Add missing DNNL_EXPERIMENTAL_UKERNEL ifdef

Add a missing `#ifdef DNNL_EXPERIMENTAL_UKERNEL` around `dnnl_api_utils`
group.  The templates inside that group reference `dnnl_brgemm_t`
and `dnnl_brgemm_pack_B_t` types that are guarded using the same
condition, and therefore attempting to include the file without the
define results in errors:

```
/home/mgorny/miniforge3/envs/pytorch/include/oneapi/dnnl/dnnl_ukernel.hpp:38:22: error: ‘dnnl_brgemm_t’ was not declared in this scope; did you mean ‘dnnl_stream_t’?
   38 | struct handle_traits<dnnl_brgemm_t> {
      |                      ^~~~~~~~~~~~~
      |                      dnnl_stream_t
/home/mgorny/miniforge3/envs/pytorch/include/oneapi/dnnl/dnnl_ukernel.hpp:38:35: error: template argument 1 is invalid
   38 | struct handle_traits<dnnl_brgemm_t> {
      |                                   ^
/home/mgorny/miniforge3/envs/pytorch/include/oneapi/dnnl/dnnl_ukernel.hpp:45:22: error: ‘dnnl_brgemm_pack_B_t’ was not declared in this scope
   45 | struct handle_traits<dnnl_brgemm_pack_B_t> {
      |                      ^~~~~~~~~~~~~~~~~~~~
/home/mgorny/miniforge3/envs/pytorch/include/oneapi/dnnl/dnnl_ukernel.hpp:45:42: error: template argument 1 is invalid
   45 | struct handle_traits<dnnl_brgemm_pack_B_t> {
      |                                          ^
```

include/oneapi/dnnl/dnnl_ukernel.hpp

@@ -29,6 +29,8 @@
 /// oneDNN namespace
 namespace dnnl {
 
+#ifdef DNNL_EXPERIMENTAL_UKERNEL
+
 /// @addtogroup dnnl_api_utils
 /// @{
 
@@ -59,6 +61,8 @@ struct handle_traits<dnnl_ukernel_attr_params_t> {
 
 /// @} dnnl_api_utils
 
+#endif
+
 /// @addtogroup dnnl_api_ukernel Ukernels
 /// Collection of ukernels
 /// @{