capsicum-test: include SCTP tests

HAVE_SCTP is #defined in syscalls.h, so move the #ifdef after the
inclusion of the header that #defines it.

Issue https://github.com/google/capsicum-test/issues/60 is open upstream
in case a slightly different version is needed for Linux.

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D46995