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.

servers/rendering/environment/renderer_fog.h

@@ -50,4 +50,4 @@ public:
 	virtual RS::FogVolumeShape fog_volume_get_shape(RID p_fog_volume) const = 0;
 };
 
-#endif // !RENDERER_FOG_H
+#endif // RENDERER_FOG_H

servers/rendering/environment/renderer_gi.h

@@ -78,4 +78,4 @@ public:
 	virtual uint32_t voxel_gi_get_version(RID p_probe) const = 0;
 };
 
-#endif // !RENDERER_GI_H
+#endif // RENDERER_GI_H