lokdocview: Use *get_instance_private () to get private structure

Let lok_doc_view_get_instance_private () do the pointer
arithmatic. Additionally, we are saving sizeof (void*) already in
the _LOKDocView struct with this approach.

Change-Id: I6d991d5834ef15dad24acb14a1d4bbf7d03df762

include/LibreOfficeKit/LibreOfficeKitGtk.h

@@ -32,7 +32,6 @@ typedef struct _LOKDocViewPrivate LOKDocViewPrivate;
 struct _LOKDocView
 {
     GtkDrawingArea aDrawingArea;
-    LOKDocViewPrivate* priv;
 };
 
 struct _LOKDocViewClass