selftests: net: add netlink-dumps to .gitignore

Commit 55d42a0c ("selftests: net: add a test for closing
a netlink socket ith dump in progress") added a new test
but did not add it to gitignore.

Reviewed-by: Joe Damato <jdamato@fastly.com>
Link: https://patch.msgid.link/20241108004731.2979878-1-kuba@kernel.org


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