Documentation: add pyyaml to requirements.txt

Commit f061c9f ("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/[email protected]/>
I'm not sure why the pyyaml requirement disappeared in the meantime.

Fixes: f061c9f ("Documentation: Document each netlink family")
Cc: Breno Leitao <[email protected]>
Cc: Jakub Kicinski <[email protected]>
Cc: David S. Miller <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Signed-off-by: Vegard Nossum <[email protected]>
Reviewed-by: Simon Horman <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

Author: vegard

Documentation/sphinx/requirements.txt

@@ -1,3 +1,4 @@
 # jinja2>=3.1 is not compatible with Sphinx<4.0
 jinja2<3.1
 Sphinx==2.4.4
+pyyaml