Input: wacom - export battery scope

This will stop UPower from detecting the tablet as a power supply,
and using its battery status to hibernate or switch off the machine.

https://bugs.freedesktop.org/show_bug.cgi?id=70321

Signed-off-by: Bastien Nocera <hadess@hadess.net>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>