AP_AHRS: fixed examples build

using a relative include works as the object is just passed through,
not used, in this header

libraries/AP_AHRS/AP_AHRS.h

@@ -26,12 +26,13 @@
 #include <inttypes.h>
 #include <AP_Compass.h>
 #include <AP_Airspeed.h>
-#include <AP_OpticalFlow.h>
 #include <AP_GPS.h>
 #include <AP_InertialSensor.h>
 #include <AP_Baro.h>
 #include <AP_Param.h>
 
+#include "../AP_OpticalFlow/AP_OpticalFlow.h"
+
 // Copter defaults to EKF on by default, all others off
 #if APM_BUILD_TYPE(APM_BUILD_ArduCopter)
 #define AHRS_EKF_USE_DEFAULT    1