AP_Logger: Forward declare AHRS

This reduces the number of places this is implicitly included, and helps
to reduce the scope of files that get rebuilt

libraries/AP_Logger/AP_Logger.h

@@ -7,7 +7,6 @@
 #include <AP_Common/AP_Common.h>
 #include <AP_Param/AP_Param.h>
 #include <AP_InertialSensor/AP_InertialSensor.h>
-#include <AP_AHRS/AP_AHRS.h>
 #include <AP_Mission/AP_Mission.h>
 #include <AP_RPM/AP_RPM.h>
 #include <AP_Logger/LogStructure.h>
@@ -22,6 +21,9 @@
 #include "LoggerMessageWriter.h"
 
 class AP_Logger_Backend;
+class AP_AHRS;
+class AP_AHRS_View;
+struct ekf_timing;
 
 // do not do anything here apart from add stuff; maintaining older
 // entries means log analysis is easier
@@ -240,9 +242,7 @@ public:
     void Write_Power(void);
     void Write_AHRS2(AP_AHRS &ahrs);
     void Write_POS(AP_AHRS &ahrs);
-#if AP_AHRS_NAVEKF_AVAILABLE
     void Write_EKF_Timing(const char *name, uint64_t time_us, const struct ekf_timing &timing);
-#endif
     void Write_Radio(const mavlink_radio_t &packet);
     void Write_Message(const char *message);
     void Write_MessageF(const char *fmt, ...);