expo: Add forward declaration for udevice to cedit

Some files may include this header file without first including dm.h
so add a forward declaration.

Signed-off-by: Simon Glass <sjg@chromium.org>

include/cedit.h

@@ -14,6 +14,7 @@
 struct abuf;
 struct expo;
 struct scene;
+struct udevice;
 struct video_priv;
 struct udevice;