selftests: mptcp: add missing kconfig for NF Mangle

Since the commit mentioned below, 'mptcp_join' selftests is using
IPTables to add rules to the Mangle table, only in IPv4.

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: b6e074e171bc ("selftests: mptcp: add infinite map testcase")
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_ADVANCED_ROUTER=y
 CONFIG_IP_MULTIPLE_TABLES=y
 CONFIG_IP_NF_FILTER=m
+CONFIG_IP_NF_MANGLE=m
 CONFIG_IP_NF_TARGET_REJECT=m
 CONFIG_IPV6_MULTIPLE_TABLES=y
 CONFIG_IP6_NF_FILTER=m