Blackbox all whiteboxes after synthesis

This prevents issues like processes in whiteboxes triggering an error in
the JSON backend.

Signed-off-by: gatecat <gatecat@ds0.me>

techlibs/xilinx/synth_xilinx.cc

@@ -662,6 +662,7 @@ struct SynthXilinxPass : public ScriptPass
 			run("hierarchy -check");
 			run("stat -tech xilinx");
 			run("check -noinit");
+			run("blackbox =A:whitebox");
 		}
 
 		if (check_label("edif")) {