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.

platform/ios/export/export_plugin.h

@@ -290,4 +290,4 @@ public:
 	}
 };
 
-#endif
+#endif // IOS_EXPORT_PLUGIN_H

platform/ios/export/godot_plugin_config.h

@@ -129,4 +129,4 @@ struct PluginConfigIOS {
 	static PluginConfigIOS load_plugin_config(Ref<ConfigFile> config_file, const String &path);
 };
 
-#endif // GODOT_PLUGIN_CONFIG_H
+#endif // IOS_GODOT_PLUGIN_CONFIG_H