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: c17cd24959cd ("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>

drivers/target/target_core_fabric_configfs.c

@@ -623,8 +623,6 @@ static struct configfs_attribute *target_fabric_port_attrs[] = {
 	NULL,
 };
 
-extern struct configfs_item_operations target_core_dev_item_ops;
-
 static int target_fabric_port_link(
 	struct config_item *lun_ci,
 	struct config_item *se_dev_ci)

drivers/target/target_core_internal.h

@@ -88,6 +88,7 @@ int	target_for_each_device(int (*fn)(struct se_device *dev, void *data),
 			       void *data);
 
 /* target_core_configfs.c */
+extern struct configfs_item_operations target_core_dev_item_ops;
 void	target_setup_backend_cits(struct target_backend *);
 
 /* target_core_fabric_configfs.c */