zoom-manager: bring back attribute aligned(8)

This was needed for some other architectures, even though it is problematic
that we have to do it (it should really be the default for the parent
GObject struct).

lib/sp-zoom-manager.c

@@ -32,7 +32,7 @@ struct _SpZoomManager
   gdouble min_zoom;
   gdouble max_zoom;
   gdouble zoom;
-};
+} __attribute__((aligned(8)));
 
 enum {
   PROP_0,