control_allocator: add helicopter mixer

Same logic as the existing mixer.
Untested.

src/modules/control_allocator/ControlAllocator.cpp

@@ -254,6 +254,10 @@ ControlAllocator::update_effectiveness_source()
 			tmp = new ActuatorEffectivenessCustom(this);
 			break;
 
+		case EffectivenessSource::HELICOPTER:
+			tmp = new ActuatorEffectivenessHelicopter(this);
+			break;
+
 		default:
 			PX4_ERR("Unknown airframe");
 			break;