Documentation: add pyyaml to requirements.txt

Commit f061c9f7 ("Documentation: Document each netlink family") added
a new Python script that is invoked during 'make htmldocs' and which reads
the netlink YAML spec files.

Using the virtualenv from scripts/sphinx-pre-install, we get this new
error wen running 'make htmldocs':

  Traceback (most recent call last):
    File "./tools/net/ynl/ynl-gen-rst.py", line 26, in <module>
      import yaml
  ModuleNotFoundError: No module named 'yaml'
  make[2]: *** [Documentation/Makefile:112: Documentation/networking/netlink_spec/rt_link.rst] Error 1
  make[1]: *** [Makefile:1708: htmldocs] Error 2

Fix this by adding 'pyyaml' to requirements.txt.

Note: This was somehow present in the original patch submission:
<https://lore.kernel.org/all/20231103135622.250314-1-leitao@debian.org/>
I'm not sure why the pyyaml requirement disappeared in the meantime.

Fixes: f061c9f7

 ("Documentation: Document each netlink family")
Cc: Breno Leitao <leitao@debian.org>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: David S. Miller <davem@davemloft.net>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>