Fix compiler warning under Clang

Summary:
Issue:
In file included from .../kcoreaddons/src/lib/caching/kshareddatacache.cpp:25:
.../kcoreaddons/src/lib/caching/kshareddatacache_p.h:169:47: warning: unknown attribute 'artificial' ignored [-Wunknown-attributes]

__attribute__((always_inline, gnu_inline, artificial))
                                          ^

Reviewers: apol

Reviewed By: apol

Subscribers: apol, Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D12112