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