Make sock_diag_test.py pass on 4.9 devices that don't have SCTP.

The test code currently checks that we can see marks set on SCTP
sockets. This doesn't work on devices that don't have SCTP. This
has never been a problem on VM tests because the VM tests enable
the SCTP config option.

Bug: 36510518
Test: sock_diag_test.py passes on android-4.9
Test: sock_diag tests pass on 4.9 device
Change-Id: I82fd77939be549a0b2c6a2ab0395b66e7b2bdc80