security/kleopatra: Fix build against libc++18

libc++ 18 by default removes unique() in 'std::shared_ptr' (in <memory>), as
a follow-up to this resolution:
https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0521r0.html

An alternative would be to get it back by defining
_LIBCPP_ENABLE_CXX20_REMOVED_SHARED_PTR_UNIQUE before including the libc++
headers, but trying to fight the standard may not be a wise move going forward,
especially if/as other C++ libraries follow suit.

Upstream MR (approved) is at
https://invent.kde.org/pim/kleopatra/-/merge_requests/205.

PR: 279139
Approved by: kde (maintainer)
Approved by: jrm (ports)
Approved by: markj (mentor)