Fix `__pattern_partial_sort_copy` forward declaration

[SergeyKopienko]

Fix __pattern_partial_sort_copy forward declaration :

• we forgot to declare __pattern_partial_sort_copy with __parallel_tag as the first param.

• Instead we had two declarations of the same __pattern_partial_sort_copy :

  template <class _Tag, class _ExecutionPolicy, class _ForwardIterator, class _RandomAccessIterator, class _Compare>
  _RandomAccessIterator
  __pattern_partial_sort_copy(_Tag, _ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, _RandomAccessIterator,
                          _RandomAccessIterator, _Compare) noexcept;

This issue has been introduced in the PR https://github.com/oneapi-src/oneDPL/pull/1239