This fixes a build issue with make -j6+ due to qapi-generated files
being built before $(GENERATED_HEADERS) have been created.
Tested-by: NStefan Berger <stefanb@linux.vnet.ibm.com>
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>