selftest: net: add tun to .gitignore

Add missing .gitignore entry.

Fixes: 839b92fe ("selftest: tun: add test for NAPI dismantle")
Link: https://lore.kernel.org/r/20220709024141.321683-1-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>