configure: Check for a sctp struct instead of just the header

This fixes build failures on debian/kfreebsd, which has the
sctp.h header, but it is currently broken (a cpp test succeeds,
but a compile test fails), see http://bugs.debian.org/684330 for
details.

Also remove the checked item from HAVE_LIST, since the corresponding
HAVE_* define isn't used by the source code.

Signed-off-by: Martin Storsjö <martin@martin.st>
