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>