build: include qapi-generated/ files in qga/Makefile.objs

No reason to leave them out, and it will ensure that the dependencies
are picked up.  Later we can perhaps move the files to another
directory to avoid ../ usage.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>