build: use pythondir for yajsonrpc

Change-Id: I7c98abc9f52f948d4807d4ee6eda6efba6c42bcd
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/17131
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-by: Eyal Edri <eedri@redhat.com>
Tested-by: Eyal Edri <eedri@redhat.com>