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: 523514ed0a99 ("selftests: mptcp: add ADD_ADDR IPv6 test cases")
Cc: [e-mail]
Reviewed-by: Geliang Tang <[e-mail]>
Signed-off-by: Matthieu Baerts (NGI0) <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

tools/testing/selftests/net/mptcp/config

···
 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