selftests: add the missing CONFIG_IP_SCTP in net config

The selftest sctp_vrf needs CONFIG_IP_SCTP set in config
when building the kernel, so add it.

Fixes: a61bd7b9 ("selftests: add a selftest for sctp vrf")
Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Reviewed-by: Sridhar Samudrala <sridhar.samudrala@intel.com>
Link: https://lore.kernel.org/r/61dddebc4d2dd98fe7fb145e24d4b2430e42b572.1681312386.git.lucien.xin@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>