GCS_MAVLink: move use of AP_AHRS_NAVEKF_AVAILABLE into cpp file

This is defined in AP_AHRS which we only get transitively in the header

libraries/GCS_MAVLink/GCS.h

@@ -420,9 +420,7 @@ public:
     virtual void send_simstate() const;
     void send_ahrs();
     void send_battery2();
-#if AP_AHRS_NAVEKF_AVAILABLE
     void send_opticalflow();
-#endif
     virtual void send_attitude() const;
     void send_autopilot_version() const;
     void send_extended_sys_state() const;

libraries/GCS_MAVLink/GCS_Common.cpp

@@ -2368,12 +2368,12 @@ MAV_RESULT GCS_MAVLINK::_set_mode_common(const MAV_MODE _base_mode, const uint32
     return result;
 }
 
-#if AP_AHRS_NAVEKF_AVAILABLE
 /*
   send OPTICAL_FLOW message
  */
 void GCS_MAVLINK::send_opticalflow()
 {
+#if AP_AHRS_NAVEKF_AVAILABLE
     const OpticalFlow *optflow = AP::opticalflow();
 
     // exit immediately if no optical flow sensor or not healthy
@@ -2407,8 +2407,8 @@ void GCS_MAVLINK::send_opticalflow()
         hagl,  // ground distance (in meters) set to zero
         flowRate.x,
         flowRate.y);
-}
 #endif
+}
 
 /*
   send AUTOPILOT_VERSION packet