sbp-target: Remove a superfluous forward declaration

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Acked-by: Chris Boot <bootc@bootc.net>
Reviewed-by: Andy Grover <agrover@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>