Flag MidiPermissionDescriptor definition as non normative

Spec prose makes it clear that the IDL is defined in the Permissions spec, but
tools that crawl specs to extract things from them do not read the prose and
happily think that the Web Midi API spec re-defines the dictionary.

index.html

@@ -170,7 +170,7 @@ <h2>
             [=powerful feature/permission descriptor type=]
           </dt>
           <dd>
-            <pre class="idl">
+            <pre class="idl exclude">
               dictionary MidiPermissionDescriptor : PermissionDescriptor {
                 boolean sysex = false;
               };