lokdocview: mark *_get_type() with const function attribute

Use G_GNUC_CONST that adds const function attribute to
lok_doc_view_get_type() for better performance.

Change-Id: Id79f0395a98c4f98b46303e9b5ee1e103fbe331f

include/LibreOfficeKit/LibreOfficeKitGtk.h

@@ -44,7 +44,7 @@ struct _LOKDocViewClass
   void (* part_changed) (LOKDocView* pView, int new_part);
 };
 
-GType                          lok_doc_view_get_type               (void);
+GType                          lok_doc_view_get_type               (void) G_GNUC_CONST;
 GtkWidget*                     lok_doc_view_new                    (LibreOfficeKit* pOffice );
 gboolean                       lok_doc_view_open_document          (LOKDocView* pDocView,
                                                                     char* pPath);