Akonadi: fix compilation with clang

Summary:
abi::__forced_unwind is gcc specific, apparently.
https://github.com/gabime/spdlog/issues/549

Test Plan: Builds

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D21077