rwm: add missing decoration hint

I forgot that the xdg-decoration protocol allows clients to specify
lack of preference.

 protocol/river-window-management-v1.xml | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/protocol/river-window-management-v1.xml b/protocol/river-window-management-v1.xml
index af618a3..fd6ed4b 100644
--- a/protocol/river-window-management-v1.xml
+++ b/protocol/river-window-management-v1.xml
@@ -364,6 +364,8 @@
         summary="client side decoration preferred, both CSD and SSD supported"/>
       <entry name="prefers_ssd" value="2"
         summary="server side decoration preferred, both CSD and SSD supported"/>
+      <entry name="no_preference" value="3"
+        summary="no preference, both CSD and SSD supported"/>
     </enum>
 
     <event name="decoration_hint">