Make svg images with links valid

* lisp/svg.el (svg-create): Specify xlink namespace for svg images
(bug#40010).

Copyright-paperwork-exempt: yes

lisp/svg.el

@@ -70,7 +70,8 @@ any further elements added."
 	      (height . ,height)
 	      (version . "1.1")
 	      (xmlns . "http://www.w3.org/2000/svg")
-	      ,@(svg--arguments nil args))))
+              (xmlns:xlink . "http://www.w3.org/1999/xlink")
+              ,@(svg--arguments nil args))))
 
 (defun svg-gradient (svg id type stops)
   "Add a gradient with ID to SVG.