keybindings: Do not depend on linux headers for above-tab key

Commit 2f229c3928be3a removed the code to compute the above-tab
keycode and replaced it with a simple constant from linux/input.h.
We obviously cannot depend on linux headers on non-linux systems,
so provide a fallback definition in that case (which is expected
to work assuming the system is using the Xorg xf86-input-keyboard
driver).

https://bugzilla.gnome.org/show_bug.cgi?id=737135