[SCSI] Make scsi_transport_spi.h includable by itself

Add forward declarations to allow scsi_transport_spi.h to be compiled by
itself.
Signed-off-by: NMatthew Wilcox <matthew@wil.cx>
Signed-off-by: NJames Bottomley <James.Bottomley@SteelEye.com>

include/scsi/scsi_transport_spi.h

@@ -24,6 +24,9 @@
 #include <linux/transport_class.h>
 
 struct scsi_transport_template;
+struct scsi_target;
+struct scsi_device;
+struct Scsi_Host;
 
 struct spi_transport_attrs {
 	int period;		/* value in the PPR/SDTR command */