target: Fix compile warning w/ missing module.h include

This patch fixes the following compile warning in target_core_cdb.c in
recent linux-next code due to the new use of EXPORT_SYMBOL() for
target_get_task_cdb().

drivers/target/target_core_cdb.c:1316: warning: data definition has no type or storage class
drivers/target/target_core_cdb.c:1316: warning: type defaults to ‘int’ in declaration of ‘EXPORT_SYMBOL’
drivers/target/target_core_cdb.c:1316: warning: parameter names (without types) in function declaration

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