Fix the build on !Linux after ef1afc0f.
Fix the build on !Linux after ef1afc0f.

akonadi_shared needs ${AKONADI_SYSTEM_LIBS} when execinfo is not in
libc, otherwise linking will fail due to backtrace(3) and others not
being found.

REVIEW: 103111

kolab-groupware/akonadi,kolab-groupware/akonadi,kolab-groupware/akonadi,kolab-groupware/akonadi