selftests: fix netfilter path in Makefile

Netfilter tests have been moved to a subdir under selftests/net by
patch series [1]. Fix the path in selftests/Makefile accordingly.

This helps fix the following error:

    tools/testing/selftests$ make
    ...
    make[1]: Entering directory 'tools/testing/selftests'
    make[1]: *** netfilter: No such file or directory.  Stop.
    make[1]: Leaving directory 'tools/testing/selftests'

[1] https://lore.kernel.org/all/20240411233624.8129-1-fw@strlen.de/



Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Yujie Liu <yujie.liu@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>