stream: make SCTP usage conditional on HAVE_LIBSCTP
Guard SCTP-specific includes and code paths with HAVE_LIBSCTP to allow building libosmo-netif without libsctp support.
This avoids unconditional inclusion of <osmocom/netif/sctp.h> and ensures that SCTP-specific variables and send paths are only compiled when SCTP support is enabled.
No functional change when HAVE_LIBSCTP is defined.
