scm: common -- Hide struct fd_opts argument

When SCM_FDSET_HAS_OPTS is not set the scm-code.c
can't be built because it declares struct fd_opts
in parameters. Lets rather hide this type and
allow to build without SCM_FDSET_HAS_OPTS definition.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>