selftests: drv-net: remove an unnecessary libmnl include

ncdevmem doesn't need libmnl, remove the unnecessary include.

Since YNL doesn't depend on libmnl either, any more, it's actually
possible to build selftests without having libmnl installed.

Reviewed-by: Mina Almasry <almasrymina@google.com>
Reviewed-by: Joe Damato <jdamato@fastly.com>
Link: https://patch.msgid.link/20250207183119.1721424-1-kuba@kernel.org


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