AP_InertialNav: remove use of AP_AHRS from most headers

don't need to know the details, just that it is a class

libraries/AP_InertialNav/AP_InertialNav.cpp

@@ -1,7 +1,7 @@
-#include <AP_HAL/AP_HAL.h>
-#include <AP_Baro/AP_Baro.h>
 #include "AP_InertialNav.h"
 
+#include <AP_AHRS/AP_AHRS.h>
+
 /*
   A wrapper around the AP_InertialNav class which uses the NavEKF
   filter if available, and falls back to the AP_InertialNav filter

libraries/AP_InertialNav/AP_InertialNav.h

@@ -5,14 +5,13 @@
  */
 #pragma once
 
-#include <AP_AHRS/AP_AHRS.h>
 #include <AP_NavEKF/AP_Nav_Common.h>              // definitions shared by inertial and ekf nav filters
 
 class AP_InertialNav
 {
 public:
     // Constructor
-    AP_InertialNav(AP_AHRS &ahrs) :
+    AP_InertialNav(class AP_AHRS &ahrs) :
         _ahrs_ekf(ahrs)
         {}