Filter: silence warning about unused function

../../libraries/Filter/examples/Derivative/Derivative.cpp:16:14: warning: ‘float noise()’ defined but not used [-Wunused-function]
 static float noise(void)
              ^

libraries/Filter/examples/Derivative/Derivative.cpp

@@ -8,6 +8,8 @@
 
 const AP_HAL::HAL& hal = AP_HAL::get_HAL();
 
+#define USE_NOISE 0
+
 DerivativeFilter<float,11> derivative;
 
 // setup routine
@@ -15,7 +17,11 @@ void setup(){}
 
 static float noise(void)
 {
+#if USE_NOISE
     return ((random() % 100)-50) * 0.001f;
+#else
+    return 0;
+#endif
 }
 
 //Main loop where the action takes place
@@ -24,7 +30,7 @@ void loop()
     hal.scheduler->delay(50);
     float t = AP_HAL::millis()*1.0e-3f;
     float s = sinf(t);
-    //s += noise();
+    s += noise();
     uint32_t t1 = AP_HAL::micros();
     derivative.update(s, t1);
     float output = derivative.slope() * 1.0e6f;