AP_OpticalFlow: fix example when OpticalFlow compiled out

Author: peterbarker

libraries/AP_OpticalFlow/examples/AP_OpticalFlow_test/AP_OpticalFlow_test.cpp

@@ -31,20 +31,26 @@ class DummyVehicle {
 };
 
 static DummyVehicle vehicle;
+#if AP_OPTICALFLOW_ENABLED
 static OpticalFlow optflow;
+#endif
 
 void setup()
 {
     hal.console->printf("OpticalFlow library test ver 1.6\n");
 
     hal.scheduler->delay(1000);
 
+#if AP_OPTICALFLOW_ENABLED
     // flowSensor initialization
     optflow.init(-1);
 
     if (!optflow.healthy()) {
-        hal.console->printf("Failed to initialise PX4Flow ");
+        hal.console->printf("Failed to initialise OpticalFlow");
     }
+#else
+    hal.console->printf("OpticalFlow compiled out");
+#endif
 
     hal.scheduler->delay(1000);
 }