tools: ynl: remove __pycache__ during clean

Build process uses python to generate the user space code.
Remove __pycache__ on make clean.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>