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 #1239

[MikeDvorskiy]

LGTM

[dmitriy-sobolev]

LGTM