Allow comma before brace for named enum

[wutka-vandy]

In cparser.mly, an enum is only allowed to have a comma directly before a } if the enum doesn't specify a type name, but if the enum does specify a type name, it throws a Cparser.MenhirBasics.Error. For example, in this file, the first enum declaration succeeds but the second causes an error (comment out the second one and see that the error disappears):

enum {
  FOO,
  BAR,
  BAZ,
};

enum Fooble {
  QUUX,
  FRED,
};

This change allows the second one to parse successfully.

[ivg]

Thank you!