AP_AHRS: move active_accel_instance into DCM private header section

libraries/AP_AHRS/AP_AHRS_Backend.h

@@ -453,7 +453,4 @@ protected:
     float _sin_roll;
     float _sin_pitch;
     float _sin_yaw;
-
-    // which accelerometer instance is active
-    uint8_t _active_accel_instance;
 };

libraries/AP_AHRS/AP_AHRS_DCM.h

@@ -214,6 +214,9 @@ private:
     float _ra_deltat;
     uint32_t _ra_sum_start;
 
+    // which accelerometer instance is active
+    uint8_t _active_accel_instance;
+
     // the earths magnetic field
     float _last_declination;
     Vector2f _mag_earth{1, 0};