target: Make spc_parse_naa_6h_vendor_specific non static

This patch makes spc_parse_naa_6h_vendor_specific() available to
other target code, which is required by EXTENDED_COPY when comparing
the received NAA WWN device identifer for locating the associated
se_device backend.

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>