i3-dmenu-desktop: Support symlinks

follow_fast is passed to find() in order to support this. Since we check
ourselves for duplicates, the fast option can be used.

Fixes #3973

i3-dmenu-desktop

@@ -143,6 +143,7 @@ find(
             $desktops{$relative} = $File::Find::name;
         },
         no_chdir => 1,
+        follow_fast => 1,
     },
     @searchdirs
 );