Changes

stream: Fix build with --disable-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.

Change-Id: I07ef25f3fcc39f9bee023d9264a72c4381b82b65