thinkpad-acpi: Add KEY_CAMERA (Fn-F6) for Lenovo keyboards

On the T410s and most likely other current models, Fn-F6 is labeled as
Camera/Headphone key.  Report key presses as KEY_CAMERA.

Signed-off-by: Jens Taprogge <jens.taprogge@taprogge.org>
Acked-by: Jerone Young <jerone.young@canonical.com>
Acked-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Matthew Garrett <mjg@redhat.com>