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.

drivers/png/png_driver_common.h

@@ -43,4 +43,4 @@ Error png_to_image(const uint8_t *p_source, size_t p_size, bool p_force_linear,
 Error image_to_png(const Ref<Image> &p_image, Vector<uint8_t> &p_buffer);
 } // namespace PNGDriverCommon
 
-#endif
+#endif // PNG_DRIVER_COMMON_H