tools: ynl: check for membership with 'not in'

tools: ynl: check for membership with 'not in'

It is better to use 'not in' instead of 'not {element} in {collection}'
according to Ruff.

This is linked to Ruff error E713 [1]:

  Testing membership with {element} not in {collection} is more readable.

Link: https://docs.astral.sh/ruff/rules/not-in-test/
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Reviewed-by: Asbjørn Sloth Tønnesen <ast@fiberby.net>
Link: https://patch.msgid.link/20250909-net-next-ynl-ruff-v1-8-238c2bccdd99@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>