There are a few multi-touch devices that support finger tracking
well in hardware, Stantum being the prime example. By exposing the
tracking ID in the MT protocol, evdev bandwidth and cpu usage in
user space can be reduced.

This patch adds the ABS_MT_TRACKING_ID to the MT protocol.
Signed-off-by: Henrik Rydberg <rydberg@euromail.se>
Tested-by: Stéphane Chatty <chatty@enac.fr>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>