Copter: Remove DRIFT mode code if DRIFT mode is disabled.

ArduCopter/mode_drift.cpp

@@ -1,5 +1,7 @@
 #include "Copter.h"
 
+#if MODE_DRIFT_ENABLED == ENABLED
+
 /*
  * Init and run calls for drift flight mode
  */
@@ -124,3 +126,4 @@ float Copter::ModeDrift::get_throttle_assist(float velz, float pilot_throttle_sc
     
     return constrain_float(pilot_throttle_scaled + thr_assist, 0.0f, 1.0f);
 }
+#endif