Docs: Add cpio requirement to changes.rst

The install target requires cpio to run the `kernel/gen_kheaders.sh`
script, but it's missing in the requirements list at
'Documentation/process/changes.rst'. This patch adds it to the list.

Signed-off-by: Bruno Moreira-Guedes <codeagain@codeagain.dev>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>