selftests: mptcp: depend on SYN_COOKIES

MPTCP selftests are using TCP SYN Cookies for quite a while now, since
v5.9.

Some CIs don't have this config option enabled and this is causing
issues in the tests:

  # ns1 MPTCP -> ns1 (10.0.1.1:10000      ) MPTCP     (duration   167ms) sysctl: cannot stat /proc/sys/net/ipv4/tcp_syncookies: No such file or directory
  # [ OK ]./mptcp_connect.sh: line 554: [: -eq: unary operator expected

There is no impact in the results but the test is not doing what it is
supposed to do.

Fixes: fed61c4b

 ("selftests: mptcp: make 2nd net namespace use tcp syn cookies unconditionally")
Cc: stable@vger.kernel.org
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: David S. Miller <davem@davemloft.net>