notification_plugin_deinit: Set function signature to int(void *)

Previously the function signature was inconsistent throughout
src/fe-gtk/plugin-notification.{h,c}: One file had the signature
int(void), while the other had int(void *). Since this type mismatch
might lead to problems (especially with LTO) and the (possibly provided)
function argument isn't used in the function's definition, this commit
sets int(void *) as function signature for both the declaration and
definition of the function.

Fixes: https://github.com/hexchat/hexchat/issues/2726

src/fe-gtk/plugin-notification.c

@@ -246,7 +246,7 @@ notification_plugin_init (hexchat_plugin *plugin_handle, char **plugin_name, cha
 
 
 int
-notification_plugin_deinit (void)
+notification_plugin_deinit (void *unused_param)
 {
 	notification_backend_deinit ();
 	return 1;