This fixes a build issue with make -j6+ due to qapi-generated files
being built before $(GENERATED_HEADERS) have been created.

Tested-by: Stefan Berger <stefanb@linux.vnet.ibm.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>