selftests: mptcp: add missing kconfig for NF Filter in v6

Since the commit mentioned below, 'mptcp_join' selftests is using
IPTables to add rules to the Filter table for IPv6.

It is then required to have IP6_NF_FILTER KConfig.

This KConfig is usually enabled by default in many defconfig, but we
recently noticed that some CI were running our selftests without them
enabled.

Fixes: 523514ed ("selftests: mptcp: add ADD_ADDR IPv6 test cases")
Cc: stable@vger.kernel.org
Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://lore.kernel.org/r/20240131-upstream-net-20240131-mptcp-ci-issues-v1-3-4c1c11e571ff@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/mptcp/config

@@ -25,6 +25,7 @@ CONFIG_IP_MULTIPLE_TABLES=y
 CONFIG_IP_NF_FILTER=m
 CONFIG_IP_NF_TARGET_REJECT=m
 CONFIG_IPV6_MULTIPLE_TABLES=y
+CONFIG_IP6_NF_FILTER=m
 CONFIG_NET_ACT_CSUM=m
 CONFIG_NET_ACT_PEDIT=m
 CONFIG_NET_CLS_ACT=y