Don't add url type if link is relative.

* org-html.el (org-html-make-link): Don't add url type if link
is relative.

lisp/org-html.el

@@ -640,7 +640,7 @@ MAY-INLINE-P allows inlining it as an image."
 	 (setq thefile 
 	    (let
 	       ((str (org-export-html-format-href thefile)))
-	       (if type
+	      (if (and type (string-match-p "^//" str))
 		  (concat type ":" str)
 		  str)))