Don't rely on sys/_types.h including sys/cdefs.h
These headers relied in BEGIN_DECS/END_DECLS being defined when
sys/_types.h was included, but there's not a requirement that this be
the case.
Reviewed by: imp
Exp-run by: antoine (PR 286274)
Pull Request: https://github.com/freebsd/freebsd-src/pull/1595