Fix build on FreeBSD by linking to Akonadi

Akonadi may require exceptions; if so, these are in its interface
target properties. Because kmail (and kmailpart) include Akonadi
headers, but did not link to it directly (only indirectly through
private linkage -- to kmailprivate and then to Akonadi --
the necessary compile options were not pulled in.

Fix this by explicitly (but privately) linking to Akonadi.