Input: add BTN_TOOL_QUINTTAP for reporting 5 fingers on touchpad

"4-finger scroll" is a gesture supported by some applications and
operating systems.

"Resting thumb" is when a clickpad user rests a finger (e.g., a
thumb), in a "click zone" (typically the bottom of the touchpad) in
anticipation of click+move=select gestures.

Thus, "4-finger scroll + resting thumb" is a 5-finger gesture.
To allow userspace to detect this gesture, we send BTN_TOOL_QUINTTAP.

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Acked-by: Chase Douglas <chase.douglas@canonical.com>
Acked-by: Henrik Rydberg <rydberg@euromail.se>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>