target: Minimize #include directives

Remove superfluous #include directives from the include/target/*.h
files. Add missing #include directives to other *.h and *.c files.
Use forward declarations for structures where possible. This
change reduces the build time for make M=drivers/target on my
laptop from 27.1s to 18.7s or by about 30%.

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Bryant G. Ly <bryantly@linux.vnet.ibm.com>
diff --git a/drivers/target/iscsi/iscsi_target_transport.c b/drivers/target/iscsi/iscsi_target_transport.c
index 08217d6..c4eb141 100644
--- a/drivers/target/iscsi/iscsi_target_transport.c
+++ b/drivers/target/iscsi/iscsi_target_transport.c
@@ -1,5 +1,6 @@
 #include <linux/spinlock.h>
 #include <linux/list.h>
+#include <linux/module.h>
 #include <target/iscsi/iscsi_transport.h>
 
 static LIST_HEAD(g_transport_list);