dselect: Do not forward declare ‘enum quitaction’

Standard C++ does not allow declaring an incomplete enum type. And the
enum is already declared in «dselect/dselect.h» which gets included
first.

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

dselect/bindings.h

@@ -21,8 +21,6 @@
 #ifndef BINDINGS_H
 #define BINDINGS_H
 
-enum quitaction;
-
 struct keybindings {
   struct interpretation;