selftests: mptcp: add missing kconfig for NF Filter

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

It is then required to have IP_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: 8d014eaa ("selftests: mptcp: add ADD_ADDR timeout test case")
Cc: stable@vger.kernel.org
Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>