Copter: explicitly ask for always using EKF

When instantiating AP_AHRS_NavEKF for ArduCopter, explicitly pass the
flag to always use the EKF.

The motivation is to move vehicle specifc code out of the general
libraries. This patch shouldn't change behavior.

ArduCopter/Copter.h

@@ -183,7 +183,7 @@ private:
     // Inertial Navigation EKF
     NavEKF EKF{&ahrs, barometer, sonar};
     NavEKF2 EKF2{&ahrs, barometer, sonar};
-    AP_AHRS_NavEKF ahrs{ins, barometer, gps, sonar, EKF, EKF2};
+    AP_AHRS_NavEKF ahrs{ins, barometer, gps, sonar, EKF, EKF2, AP_AHRS_NavEKF::FLAG_ALWAYS_USE_EKF};
 
 #if CONFIG_HAL_BOARD == HAL_BOARD_SITL
     SITL sitl;