dselect: Change keybindings forward declaration to a struct

This matches the type of the declaration in «dselect/bindings.h».

This was spotted by a run with the clang static analyzer.

dselect/dselect.h

@@ -36,7 +36,7 @@ struct helpmenuentry {
   const struct helpmessage *msg;
 };
 
-class keybindings;
+struct keybindings;
 
 class baselist {
 protected: