Add missing netfilter config dependency.

Fixes following example error when running tests via tdc.sh for all XT
tests:

 # $ sudo ./tdc.py -d eth2 -e 2029
 # Test 2029: Add xt action with log-prefix
 # exit: 255
 # exit: 0
 #  failed to find target LOG
 #
 # bad action parsing
 # parse_action: bad value (7:xt)!
 # Illegal "action"
 #
 # -----> teardown stage *** Could not execute: "$TC actions flush action xt"
 #
 # -----> teardown stage *** Error message: "Error: Cannot flush unknown TC action.
 # We have an error flushing
 # "
 # returncode 1; expected [0]
 #
 # -----> teardown stage *** Aborting test run.
 #
 # <_io.BufferedReader name=3> *** stdout ***
 #
 # <_io.BufferedReader name=5> *** stderr ***
 # "-----> teardown stage" did not complete successfully
 # Exception <class '__main__.PluginMgrTestFail'> ('teardown', ' failed to find target LOG\n\nbad action parsing\nparse_action: bad value (7:xt)!\nIllegal "action"\n', '"-----> teardown stage" did not complete successfully') (caught in test_runner, running test 2 2029 Add xt action with log-prefix stage teardown)
 # ---------------
 # traceback
 #   File "/images/src/linux/tools/testing/selftests/tc-testing/./tdc.py", line 495, in test_runner
 #     res = run_one_test(pm, args, index, tidx)
 #   File "/images/src/linux/tools/testing/selftests/tc-testing/./tdc.py", line 434, in run_one_test
 #     prepare_env(args, pm, 'teardown', '-----> teardown stage', tidx['teardown'], procout)
 #   File "/images/src/linux/tools/testing/selftests/tc-testing/./tdc.py", line 245, in prepare_env
 #     raise PluginMgrTestFail(
 # ---------------
 # accumulated output for this test:
 #  failed to find target LOG
 #
 # bad action parsing
 # parse_action: bad value (7:xt)!
 # Illegal "action"
 #
 # ---------------
 #
 # All test results:
 #
 # 1..1
 # ok 1 2029 - Add xt action with log-prefix # skipped - "-----> teardown stage" did not complete successfully

Fixes: 910d504b ("selftests/tc-testings: add selftests for xt action")
Signed-off-by: Vlad Buslov <vladbu@nvidia.com>
Reviewed-by: Pedro Tammela <pctammela@mojatatu.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>