iscsi-target: remove improper externs

These externs aren't needed and Sparse complains about them.

drivers/target/iscsi/iscsi_target_nodeattrib.c:52:12: warning:
	function 'iscsit_na_dataout_timeout' with external linkage has
	definition

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>