Made the recognizer compatible with the last API changes of libkface (cf. Bug #339524)
Polish
Depend on the latest libkface version.
cmake: unset KFACE_FOUND if not found w/ cmake module

This way can be set again by the pkg-config search module.
add missing forward declaration
cmake: fix Kface detection/usage using its cmake module
Depend on the correct version of libkface.