lockfree: spsc-queue - correct dequeue to output iterator

fixes #8629

Signed-off-by: Tim Blechmann <tim@klingt.org>

svn path=/trunk/boost/lockfree/; revision=84571
lockfree: prevent warnings for unused arguments

Signed-off-by: Tim Blechmann <tim@klingt.org>

svn path=/trunk/boost/lockfree/; revision=84618