selftests: mptcp: fix dependecies

Since commit df62f2ec ("selftests/mptcp: add diag interface tests")
the MPTCP selftests relies on the MPTCP diag interface which is
enabled by a specific kconfig knob: be sure to include it.

Fixes: df62f2ec ("selftests/mptcp: add diag interface tests")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

tools/testing/selftests/net/mptcp/config

 CONFIG_MPTCP=y
 CONFIG_MPTCP_IPV6=y
+CONFIG_INET_DIAG=m
+CONFIG_INET_MPTCP_DIAG=m
 CONFIG_VETH=y
 CONFIG_NET_SCH_NETEM=m