iscsi: fix build errors when linux/phy*.h is removed from net/dsa.h

drivers/target/iscsi/iscsi_target_login.c:1135:7: error: implicit declaration of function 'try_module_get' [-Werror=implicit-function-declaration]

Add linux/module.h to iscsi_target_login.c.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Bart Van Assche <bart.vanassche@sandisk.com>
Acked-by: Nicholas Bellinger <nab@linux-iscsi.org>
Signed-off-by: David S. Miller <davem@davemloft.net>