selftests/net: Add xt_policy config for xfrm_policy test

When running Kselftests with the current selftests/net/config
the following problem can be seen with the net:xfrm_policy.sh
selftest:

  # selftests: net: xfrm_policy.sh
  [   41.076721] IPv6: ADDRCONF(NETDEV_CHANGE): veth0: link becomes ready
  [   41.094787] IPv6: ADDRCONF(NETDEV_CHANGE): veth0: link becomes ready
  [   41.107635] IPv6: ADDRCONF(NETDEV_CHANGE): veth0: link becomes ready
  # modprobe: FATAL: Module ip_tables not found in directory /lib/modules/6.1.36
  # iptables v1.8.7 (legacy): can't initialize iptables table `filter': Table does not exist (do you need to insmod?)
  # Perhaps iptables or your kernel needs to be upgraded.
  # modprobe: FATAL: Module ip_tables not found in directory /lib/modules/6.1.36
  # iptables v1.8.7 (legacy): can't initialize iptables table `filter': Table does not exist (do you need to insmod?)
  # Perhaps iptables or your kernel needs to be upgraded.
  # SKIP: Could not insert iptables rule
  ok 1 selftests: net: xfrm_policy.sh # SKIP

This is because IPsec "policy" match support is not available
to the kernel.

This patch adds CONFIG_NETFILTER_XT_MATCH_POLICY as a module
to the selftests/net/config file, so that `make
kselftest-merge` can take this into consideration.

Signed-off-by: Daniel Díaz <daniel.diaz@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>