docs: kernel_include.py: add sphinx build dependencies

The files included by kernel-include should be added as build
dependencies, in order for sphinx-build to rebuild the corresponding
docs if any changes at the included file happens.

Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Link: https://lore.kernel.org/r/d30fc4fa422a13b7e1623d690945c46b58a55e79.1648290305.git.mchehab@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>