presentation-time: add missing bitfield marker

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Reviewed-by: Simon Ser <contact@emersion.fr>
Acked-by: Daniel Stone <daniels@collabora.com>
Acked-by: Victor Berger <victor.berger@m4x.org>

stable/presentation-time/presentation-time.xml

@@ -154,7 +154,7 @@
            summary="presentation output"/>
     </event>
 
-    <enum name="kind">
+    <enum name="kind" bitfield="true">
       <description summary="bitmask of flags in presented event">
         These flags provide information about how the presentation of
         the related content update was done. The intent is to help