scripts/get_feat.pl: allow output the parsed file names

Such output could be helpful while debugging it, but its main
goal is to tell kernel_feat.py about what files were used
by the script. Thie way, kernel_feat.py can add those as
documentation dependencies.

Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Link: https://lore.kernel.org/r/11b438ee01e00c866f5ea197d6aecc26e9f86945.1648290305.git.mchehab@kernel.org


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