target: Move a declaration of a global variable into a header file

This patch avoids that sparse reports the following warning:

drivers/target/target_core_configfs.c:2267:33: warning: symbol 'target_core_dev_item_ops' was not declared. Should it be static?

Fixes: c17cd249 ("target/configfs: Kill se_device->dev_link_magic")
Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Mike Christie <mchristi@redhat.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>