Add include guard in headers files (#8108)

Signed-off-by: Sugnan Prabhu S <sugnan.prabhu.s@intel.com>

src/lib/tailsitter_recovery/tailsitter_recovery.h

@@ -47,6 +47,9 @@
  *
  * @author Roman Bapst <bapstroman@gmail.com>
 */
+
+#pragma once
+
 #include <lib/mathlib/mathlib.h>
 
 #define SigmoidFunction(val) 1/(1 + expf(-val))

src/lib/terrain_estimation/terrain_estimator.h

@@ -35,6 +35,8 @@
  * @file terrain_estimator.h
  */
 
+#pragma once
+
 #include <lib/mathlib/mathlib.h>
 #include "matrix/Matrix.hpp"
 #include <uORB/topics/sensor_combined.h>