Clang16 will not allow incompatible pointer types by default.

Bug: https://bugs.gentoo.org/881081

Signed-off-by: Pascal Jäger <pascal.jaeger@leimstift.de>
--- a/plugin.h
+++ b/plugin.h
@@ -9,6 +9,8 @@
 #include <stdio.h>
 #include "panel.h"
 
+struct _plugin;
+
 typedef struct {
     /* common */
     char *fname;
-- 
2.38.1