Fix pretty-printing of enumerations

[nberth]

Before this change enumeration items were duplicated by the default pretty-printer.

Here's a way to check the behavior of the pretty-printer. With test.ml and x.c as follows:

#use "topfind";;
#require "goblint-cil";;
GoblintCil.dumpFile GoblintCil.defaultCilPrinter Stdlib.stdout ""
  (GoblintCil.Frontc.parse "x.c" ());;

enum e { A, B };

Before this PR, test command ocaml ./test.ml | head resulted in

/* Generated by Goblint-CIL v. %%VERSION_NUM%% */
/* print_CIL_Input is false */

#line 1 "x.c"
enum e {
    AA = 0,
    BB = 1
} ;

[michael-schwarz]

Thank you for the fix!