hw/s390x: fix build for virtio-9p-ccw

hw/s390x: fix build for virtio-9p-ccw

Commit  2c44220d05 ("meson: convert hw/arch*"), which migrated the old
Makefile.objs to meson.build  accidentally excluded virtio-ccw-9p.c and
thus the virtio-9p-ccw device from the build (and potentially also
included the file virtio-ccw-blk.c twice in the source set). And since
CONFIG_VIRTFS can't be used the way it was used here (see commit
2c9dce0196 ("meson: do not use CONFIG_VIRTFS")), the preconditions have
to be written differently.

Let's fix this!

Signed-off-by: Halil Pasic <pasic@linux.ibm.com>
Fixes: 2c44220d05 ("meson: convert hw/arch*")
Reported-by: Jakob Naucke <jakob.naucke@ibm.com>
Cc: qemu-stable@nongnu.org
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20210218034059.1096078-1-pasic@linux.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>