AP_Navigation: replace header guard with pragma once

libraries/AP_Navigation/AP_Navigation.h

@@ -9,9 +9,7 @@
   interface. All variables used by controllers should be in their
   own class.
  */
-
-#ifndef AP_NAVIGATION_H
-#define AP_NAVIGATION_H
+#pragma once
 
 #include <AP_Common/AP_Common.h>
 
@@ -110,6 +108,3 @@ public:
 		CONTROLLER_L1           = 1
 	};
 };
-
-
-#endif // AP_NAVIGATION_H