MAINTAINERS: add selftests to network drivers

tools/testing/selftests/drivers/net/ is not listed under
networking entries. Add it to NETWORKING DRIVERS.

Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20240814142832.3473685-1-kuba@kernel.org


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