Don't add repeated xlmns:xlink declarations in svg-create

* lisp/svg.el (svg-create): Fix previous unconditional addition of
the xmlns:xlink declaration -- callers may already add one, and
having it twice is something most svg libraries doesn't like.

lisp/svg.el

@@ -70,7 +70,8 @@ any further elements added."
 	      (height . ,height)
 	      (version . "1.1")
 	      (xmlns . "http://www.w3.org/2000/svg")
-              (xmlns:xlink . "http://www.w3.org/1999/xlink")
+              ,@(unless (plist-get args :xmlns:xlink)
+                  '((xmlns:xlink . "http://www.w3.org/1999/xlink")))
               ,@(svg--arguments nil args))))
 
 (defun svg-gradient (svg id type stops)