gobject: Drop volatile qualifier from GObject.ref_count

This is an API break, but no third party code should be touching
`GObject.ref_count`, let alone in a way which would be changed by the
removal of the `volatile` qualifier.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

Helps: #600

gobject/gobject.h

@@ -247,7 +247,7 @@ struct  _GObject
   GTypeInstance  g_type_instance;
   
   /*< private >*/
-  volatile guint ref_count;
+  guint          ref_count;  /* (atomic) */
   GData         *qdata;
 };
 /**