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

After commit ba1183da9a10b94611cad88c44a5c6df005f9b55 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

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>