hw/9pfs: Include virtio-9p-device.o in build

After commit ba1183da we are including
hw/Makefile.objs directly from Makefile.target. Make sure hw/Makefile.objs
rules doesn't depend on variable defined in Makefile.objs

Tested-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>