target: iscsi_target depends on NET

iscsi target code uses lots on network interface functions, so it
should depend on NET.  Fixes many build errors when NET is not enabled:

ERROR: "kernel_sendmsg" [drivers/target/iscsi/iscsi_target_mod.ko] undefined!
ERROR: "in_aton" [drivers/target/iscsi/iscsi_target_mod.ko] undefined!
ERROR: "sock_release" [drivers/target/iscsi/iscsi_target_mod.ko] undefined!
ERROR: "kernel_listen" [drivers/target/iscsi/iscsi_target_mod.ko] undefined!
ERROR: "kernel_setsockopt" [drivers/target/iscsi/iscsi_target_mod.ko] undefined!
ERROR: "kernel_recvmsg" [drivers/target/iscsi/iscsi_target_mod.ko] undefined!
ERROR: "kernel_accept" [drivers/target/iscsi/iscsi_target_mod.ko] undefined!
ERROR: "sock_create" [drivers/target/iscsi/iscsi_target_mod.ko] undefined!
ERROR: "kernel_bind" [drivers/target/iscsi/iscsi_target_mod.ko] undefined!
ERROR: "in6_pton" [drivers/target/iscsi/iscsi_target_mod.ko] undefined!

Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>