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>

Documentation/process/changes.rst

@@ -56,6 +56,7 @@ iptables               1.4.2            iptables -V
 openssl & libcrypto    1.0.0            openssl version
 bc                     1.06.95          bc --version
 Sphinx\ [#f1]_         1.7              sphinx-build --version
+cpio                   any              cpio --version
 ====================== ===============  ========================================
 
 .. [#f1] Sphinx is needed only to build the Kernel documentation
@@ -458,6 +459,11 @@ mcelog
 
 - <http://www.mcelog.org/>
 
+cpio
+----
+
+- <https://www.gnu.org/software/cpio/>
+
 Networking
 **********