docs: kernel_feat.py: add build dependencies

Ensure that the feature files will be regenerated if any
changes happen at the Documentation/features files that were
processed by gen_feat.pl.

Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Link: https://lore.kernel.org/r/5cdf7a8300019129dcc09d4c2557f75908754445.1648290305.git.mchehab@kernel.org


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