AP_OpticalFlow: example fix travis warning

missing function declaration
implicit cast
some style fix

libraries/AP_OpticalFlow/examples/AP_OpticalFlow_test/AP_OpticalFlow_test.cpp

@@ -14,6 +14,9 @@
 #include <AP_OpticalFlow/AP_OpticalFlow.h>
 #include <AP_RangeFinder/AP_RangeFinder.h>
 
+void setup();
+void loop();
+
 const AP_HAL::HAL& hal = AP_HAL::get_HAL();
 
 class DummyVehicle {