Correct deprecation syntax of oneapi::dpl::experimental::kt::esimd namespace

[mmichel11]

When including <oneapi/dpl/experimental/kernel_templates> with newer compilers, I see the following compilation error:

In file included from ../repos/oneDPL/include/oneapi/dpl/experimental/kernel_templates:18:
../repos/oneDPL/include/oneapi/dpl/experimental/kt/esimd_radix_sort.h:179:5: error: an attribute list cannot appear here
  179 |     [[deprecated("Use of oneapi::dpl::experimental::kt::esimd namespace is deprecated "
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  180 |                  "and will be removed in a future release. "
      |                  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  181 |                  "Use oneapi::dpl::experimental::kt::gpu::esimd instead")]]

According to https://en.cppreference.com/w/cpp/language/attributes/deprecated, it appears the proper syntax to deprecate a namespace is namespace [[deprecated]] NS { ... }. Our deprecation attribute is provided after the namespace's name which appears incorrect.

After correcting this, I see the proper warning when trying to compile using the deprecated namespace:

 warning: 'esimd' is deprecated: Use of oneapi::dpl::experimental::kt::esimd namespace is deprecated and will be removed in a future release. Use oneapi::dpl::experimental::kt::gpu::esimd instead [-Wdeprecated-declarations]

The placement of the attribute after the namespace seems to be a new error in clang which can be seen on trunk, an error with MSVC, and a warning with gcc when compiling with -Wpedantic: https://godbolt.org/z/xo4YfPKqo

[dmitriy-sobolev]

LGTM. Thanks for finding and fixing it.