tablet: default to a built-in tablet

tablet: default to a built-in tablet

The vast majority of devices that libwacom doesn't know about are the
various built-in ones. Since the only effect in our code here is that we
enable the calibration matrix, let's default to built-in if we don't
know any better - better to have the matrix and not use it than to not
be able to calibrate a tablet.

Note that libwacom 2.11 and later also now default to a built-in tablet.

Part-of: <https://gitlab.freedesktop.org/libinput/libinput/-/merge_requests/1074>