mouse-sel.el: Fix mouse-sel-get-selection-function

* lisp/obsolete/mouse-sel.el (mouse-sel-get-selection-function):
Use gui--last-selected-text-primary instead of no longer existing
gui-last-selected-text.

lisp/obsolete/mouse-sel.el

@@ -314,7 +314,7 @@ is `interprogram-cut-paste'.")
     (if (eq selection 'PRIMARY)
 	(or (gui-selection-value)
 	    (bound-and-true-p x-last-selected-text-primary)
-            gui-last-selected-text)
+            gui--last-selected-text-primary)
       (x-get-selection selection)))
   "Function to call to get the selection.
 Called with one argument: