Copter: land_do_not_use_GPS has no return value

ArduCopter/control_land.pde

@@ -176,7 +176,7 @@ static bool update_land_detector()
 // land_do_not_use_GPS - forces land-mode to not use the GPS but instead rely on pilot input for roll and pitch
 //  called during GPS failsafe to ensure that if we were already in LAND mode that we do not use the GPS
 //  has no effect if we are not already in LAND mode
-static bool land_do_not_use_GPS()
+static void land_do_not_use_GPS()
 {
     land_with_gps = false;
 }