Forbid enum redefinition.

Prevent the following code from compiling:

enum color {RED, GREEN, BLUE};
enum color {R, G, B};

int main()
{
        return R;
}

Reported-by: John Haque <j.eh@mchsi.com>

tccgen.c

@@ -2801,6 +2801,7 @@ static void struct_decl(CType *type, int u)
                 if (tok == '}')
                     break;
             }
+            s->c = type_size(&int_type, &align);
             skip('}');
         } else {
             maxalign = 1;