firmware: ibft depends on SCSI

Prevent build errors when SCSI is not enabled:

iscsi_ibft.c:(.init.text+0x548d): undefined reference to `iscsi_boot_create_initiator'
iscsi_ibft.c:(.init.text+0x54a9): undefined reference to `iscsi_boot_create_ethernet'
iscsi_ibft.c:(.init.text+0x54c5): undefined reference to `iscsi_boot_create_target'
iscsi_ibft.c:(.init.text+0x55ff): undefined reference to `iscsi_boot_destroy_kset'
iscsi_ibft.c:(.init.text+0x561e): undefined reference to `iscsi_boot_create_kset'
iscsi_ibft.c:(.exit.text+0xe2c): undefined reference to `iscsi_boot_destroy_kset'

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Peter Jones <pjones@redhat.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad@kernel.org>