target: Make target_core_subsystem defined as non static

This patch makes the top-level target_core_subsystem array available
to other target code, which is required by EXTENDED_COPY to pin the
backend se_device using configfs_depend_item(), in order to ensure
it can't be removed for the duration of a EXTENDED_COPY operation.

Cc: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Martin Petersen <martin.petersen@oracle.com>
Cc: Chris Mason <chris.mason@fusionio.com>
Cc: Roland Dreier <roland@purestorage.com>
Cc: Zach Brown <zab@redhat.com>
Cc: James Bottomley <JBottomley@Parallels.com>
Cc: Nicholas Bellinger <nab@linux-iscsi.org>
Signed-off-by: Nicholas Bellinger <nab@daterainc.com>