Fix build with telepathy-qt 0.9.8

Use the new TelepathyQt5::Core link target with 0.9.8, otherwise compilation fails with
/usr/include/telepathy-qt5/TelepathyQt/_gen/types.h:15:10: fatal error: QDBusArgument: No such file or directory

Differential Revision: https://phabricator.kde.org/D25370