scripts: sphinx-pre-install: add required ctex dependency

After a change meant to fix support for oriental characters
(Chinese, Japanese, Korean), ctex stylesheet is now a requirement
for PDF output.

Reported-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Link: https://lore.kernel.org/r/165aa6167f21e3892a6e308688c93c756e94f4e0.1641243581.git.mchehab@kernel.org


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