Make virtio-pci building conditional again

Commit b305b9d7 made building of virtio-pci
conditional and not enabled on S390x, because it collides with the S390 bus.

Commit 087431d1 accidentially reverted that
behavior, breaking S390x again.

So here's a follow-up patch disabling building of virtio-pci on S390x again.

This unbreaks the S390x target.
Signed-off-by: NAlexander Graf <agraf@suse.de>
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>

Makefile.target

@@ -164,7 +164,8 @@ ifdef CONFIG_SOFTMMU
 obj-y = arch_init.o cpus.o monitor.o machine.o gdbstub.o vl.o
 # virtio has to be here due to weird dependency between PCI and virtio-net.
 # need to fix this properly
-obj-y += virtio-blk.o virtio-balloon.o virtio-net.o virtio-pci.o virtio-serial-bus.o
+obj-y += virtio-blk.o virtio-balloon.o virtio-net.o virtio-serial-bus.o
+obj-$(CONFIG_VIRTIO_PCI) += virtio-pci.o
 obj-y += event_notifier.o
 obj-y += vhost_net.o
 obj-$(CONFIG_VHOST_NET) += vhost.o