selftests: mptcp: depends on built-in IPv6

Recently, CONFIG_MPTCP_IPV6 no longer selects CONFIG_IPV6. As a
consequence, if CONFIG_MPTCP_IPV6=y is added to the kconfig, it will no
longer ensure CONFIG_IPV6=y. If it is not enabled, CONFIG_MPTCP_IPV6
will stay disabled and selftests will fail.

We also need CONFIG_IPV6 to be built-in. For more details, please see
commit 0ed37ac5 ("mptcp: depends on IPV6 but not as a module").

Note that 'make kselftest-merge' will take all 'config' files found in
'tools/testsing/selftests'. Because some of them already set
CONFIG_IPV6=y, MPTCP selftests were still passing. But they will fail if
MPTCP selftests are launched manually after having executed this command
to prepare the kernel config:

  ./scripts/kconfig/merge_config.sh -m .config \
      ./tools/testing/selftests/net/mptcp/config

Fixes: 010b430d ("mptcp: MPTCP_IPV6 should depend on IPV6 instead of selecting it")
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Link: https://lore.kernel.org/r/20201021155549.933731-1-matthieu.baerts@tessares.netSigned-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/mptcp/config

 CONFIG_MPTCP=y
+CONFIG_IPV6=y
 CONFIG_MPTCP_IPV6=y
 CONFIG_INET_DIAG=m
 CONFIG_INET_MPTCP_DIAG=m