Changes

core: guard SCTP include in osmo_io_internal.h

Include <netinet/sctp.h> only when libsctp support is available.

This avoids pulling in SCTP-specific headers on builds where libsctp is
disabled or unavailable, while keeping the internal header usable across
all configurations.

No functional changes intended.

Change-Id: Icd626d908192a954d55d76a46d7cc863ed332631