It seems like (at least some versions of) gdk don't set
GDK_SOURCE_TOUCHPAD on synaptics devices. In this case though, we
don't need the additional check. Just knowing if there's any device
being driven by this X driver is enough.
https://bugzilla.gnome.org/show_bug.cgi?id=784266
Since we don't support the synaptics driver anymore we shouldn't show
the touchpad section in that case. In addition, emit a warning so that
this system mis-configuration is easier to diagnose.
https://bugzilla.gnome.org/show_bug.cgi?id=764257