[SCSI] target: remove EXTRA_CFLAGS

Add the current directory is superflous in general, and no includes in
drivers/scsi are needed either.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Nicholas A. Bellinger <nab@linux-iscsi.org>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>