AP_AHRS: replace header guard with pragma once

libraries/AP_AHRS/AP_AHRS.h

@@ -1,7 +1,6 @@
 /// -*- tab-width: 4; Mode: C++; c-basic-offset: 4; indent-tabs-mode: nil -*-
+#pragma once
 
-#ifndef __AP_AHRS_H__
-#define __AP_AHRS_H__
 /*
    This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -513,5 +512,3 @@ protected:
 #else
 #define AP_AHRS_TYPE AP_AHRS
 #endif
-
-#endif // __AP_AHRS_H__

libraries/AP_AHRS/AP_AHRS_DCM.h

@@ -1,6 +1,6 @@
 /// -*- tab-width: 4; Mode: C++; c-basic-offset: 4; indent-tabs-mode: nil -*-
-#ifndef __AP_AHRS_DCM_H__
-#define __AP_AHRS_DCM_H__
+#pragma once
+
 /*
    This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -206,5 +206,3 @@ private:
     // time when DCM was last reset
     uint32_t _last_startup_ms;
 };
-
-#endif // __AP_AHRS_DCM_H__

libraries/AP_AHRS/AP_AHRS_NavEKF.h

@@ -1,6 +1,6 @@
 /// -*- tab-width: 4; Mode: C++; c-basic-offset: 4; indent-tabs-mode: nil -*-
-#ifndef __AP_AHRS_NAVEKF_H__
-#define __AP_AHRS_NAVEKF_H__
+#pragma once
+
 /*
    This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -245,5 +245,3 @@ private:
 #endif    
 };
 #endif
-
-#endif // __AP_AHRS_NAVEKF_H__