deskutils/showdown: handle vala update

PR:             249940

deskutils/showdown/files/patch-src_showdown.vala

+--- src/showdown.vala.orig	2022-08-26 16:08:34 UTC
++++ src/showdown.vala
+@@ -11,7 +11,7 @@ class Showdown.Application: Gtk.Application {
+         {"quit", quit},
+     };
+ 
+-    public const OptionEntry[] options = {
++    const OptionEntry[] options = {
+         {
+             "open-in-current-window", 'w', 0,
+             OptionArg.FILENAME, ref wflag,