docs: kernel_abi.py: add sphinx build dependencies

Ensure that Sphinx-build will handle the files parsed by
get_abi.pl as dependencies. This way, if they are touched,
the ABI output will be regenerated.

Reported-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Link: https://lore.kernel.org/r/10bc3d3bc74f514a539cd3b48b9d287d2b6f99e2.1648290305.git.mchehab@kernel.org


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