Allow "feature" attribute in XRC schema

This is just a hack because we handle "feature" as part of "platform"
when it's not really related to it, but as it has to be treated in the
same way, it's the simplest way to allow it.

This should also have been done in 3fdebc96cd (Add support for XRC
"feature" attribute, 2023-01-29) but was forgotten there.

misc/schema/xrc_schema.rnc

@@ -430,7 +430,9 @@ stdWindowProperties =
         [xrc:p="o"] element ownfont {_, t_font }* &
         [xrc:p="o"] element help    {_, t_text }*
 
+# hack: add "feature" to "platform" because it also can be used on any element
 platform =
+        attribute feature  { t_string }? &
         attribute platform {
             xsd:string { pattern = "(win|mac|msw|unix)( *\| *(win|mac|msw|unix))*" }
         }?