Fix handling of devices with only buttons (e.g. tablet pads).

When transforming 'only buttons' device into a pseudo-mouse by adding
relative X and Y axes, EVDEV_RELATIVE_EVENTS flag  must be set on device,
otherwise it gets classified as a keyboard by X11.

https://bugs.freedesktop.org/show_bug.cgi?id=104630



Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>