kselftests/net: add missed toeplitz.sh/toeplitz_client.sh to Makefile

When generating the selftests to another folder, the toeplitz.sh
and toeplitz_client.sh are missing as they are not in Makefile, e.g.

  make -C tools/testing/selftests/ install \
      TARGETS="net" INSTALL_PATH=/tmp/kselftests

Making them under TEST_PROGS_EXTENDED as they test NIC hardware features
and are not intended to be run from kselftests.

Fixes: 5ebfb4cc ("selftests/net: toeplitz test")
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>