tools: net: add __pycache__ to gitignore

Python will generate its customary cache when running ynl scripts:

?? tools/net/ynl/lib/__pycache__/

Reported-by: Chuck Lever III <chuck.lever@oracle.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/net/ynl/lib/.gitignore

+__pycache__/