selftests: drv-net: add missing kconfig for psp.py

selftests: drv-net: add missing kconfig for psp.py

This psp.py selftest was failing on my side when only using the
drivers/net config file on top of the default one -- the recommended way
to execute selftest targets.

It looks like some kernel config are needed to execute the new tc
commands.

Note that this was not visible on NIPA, because these tests are executed
with the drivers/net/hw ones, combining the two config files, and the hw
one contains the missing ones.

Fixes: 3f74d5bb807e ("selftests/net: Add env for container based tests")
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Reviewed-by: Wei Wang <weibunny@fb.com>
Link: https://patch.msgid.link/20260710-net-sft-fix-containers-v1-6-a2915c294ef5@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>