selftests: tc: add ConnTrack procfs kconfig

When looking at the TC selftest reports, I noticed one test was failing
because /proc/net/nf_conntrack was not available.

  not ok 373 3992 - Add ct action triggering DNAT tuple conflict
  	Could not match regex pattern. Verify command output:
  cat: /proc/net/nf_conntrack: No such file or directory

It is only available if NF_CONNTRACK_PROCFS kconfig is set. So the issue
can be fixed simply by adding it to the list of required kconfig.

Fixes: e4690564 ("tc-testing: add test for ct DNAT tuple collision")
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/netdev/0e061d4a-9a23-9f58-3b35-d8919de332d7@tessares.net/T/

 [1]
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Tested-by: Zhengchao Shao <shaozhengchao@huawei.com>
Link: https://lore.kernel.org/r/20230713-tc-selftests-lkft-v1-3-1eb4fd3a96e7@tessares.net


Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>