Fix missing GDCLASS headers in the editor preview plugins

editor/plugins/editor_preview_plugins.h

@@ -70,10 +70,12 @@ public:
 };
 
 class EditorPackedScenePreviewPlugin : public EditorResourcePreviewGenerator {
+	GDCLASS(EditorPackedScenePreviewPlugin, EditorResourcePreviewGenerator);
+
 public:
-	virtual bool handles(const String &p_type) const;
-	virtual Ref<Texture2D> generate(const Ref<Resource> &p_from, const Size2 &p_size) const;
-	virtual Ref<Texture2D> generate_from_path(const String &p_path, const Size2 &p_size) const;
+	virtual bool handles(const String &p_type) const override;
+	virtual Ref<Texture2D> generate(const Ref<Resource> &p_from, const Size2 &p_size) const override;
+	virtual Ref<Texture2D> generate_from_path(const String &p_path, const Size2 &p_size) const override;
 
 	EditorPackedScenePreviewPlugin();
 };
@@ -107,17 +109,21 @@ public:
 };
 
 class EditorScriptPreviewPlugin : public EditorResourcePreviewGenerator {
+	GDCLASS(EditorScriptPreviewPlugin, EditorResourcePreviewGenerator);
+
 public:
-	virtual bool handles(const String &p_type) const;
-	virtual Ref<Texture2D> generate(const Ref<Resource> &p_from, const Size2 &p_size) const;
+	virtual bool handles(const String &p_type) const override;
+	virtual Ref<Texture2D> generate(const Ref<Resource> &p_from, const Size2 &p_size) const override;
 
 	EditorScriptPreviewPlugin();
 };
 
 class EditorAudioStreamPreviewPlugin : public EditorResourcePreviewGenerator {
+	GDCLASS(EditorAudioStreamPreviewPlugin, EditorResourcePreviewGenerator);
+
 public:
-	virtual bool handles(const String &p_type) const;
-	virtual Ref<Texture2D> generate(const Ref<Resource> &p_from, const Size2 &p_size) const;
+	virtual bool handles(const String &p_type) const override;
+	virtual Ref<Texture2D> generate(const Ref<Resource> &p_from, const Size2 &p_size) const override;
 
 	EditorAudioStreamPreviewPlugin();
 };