Code quality: Fix header guards consistency

Adds `header_guards.sh` bash script, used in CI to validate future
changes. Can be run locally to fix invalid header guards.

scene/3d/velocity_tracker_3d.h

@@ -28,8 +28,8 @@
 /* SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.                */
 /*************************************************************************/
 
-#ifndef SPATIAL_VELOCITY_TRACKER_H
-#define SPATIAL_VELOCITY_TRACKER_H
+#ifndef VELOCITY_TRACKER_3D_H
+#define VELOCITY_TRACKER_3D_H
 
 #include "scene/3d/node_3d.h"
 
@@ -58,4 +58,4 @@ public:
 	VelocityTracker3D();
 };
 
-#endif // SPATIAL_VELOCITY_TRACKER_H
+#endif // VELOCITY_TRACKER_3D_H