build: make libinput-backend a helper lib

Rather than having fbdev and drm backends include the libinput files ad hoc,
wrap them in a static library. Using the dependency object for that helper
library will then automatically pull in any necerray include dirs for the
users.

This helps with moving the backends into subdirectories.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>