Always build the public osmo_sock_multiaddr_* helper functions, independent of HAVE_LIBSCTP.
Move the HAVE_LIBSCTP checks inside the functions and limit them to the SCTP-specific code paths: when IPPROTO_SCTP is requested without libsctp support, return -ENOTSUP, while non-SCTP protocols continue to work as before.
This fixes link failures in libosmo-netif when building with --disable-libsctp.
