Report error when redefining enumerator

Prevent the following code from compiling:

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

tccgen.c

@@ -2786,6 +2786,10 @@ static void struct_decl(CType *type, int u, int tdef)
                 v = tok;
                 if (v < TOK_UIDENT)
                     expect("identifier");
+                ss = sym_find(v);
+                if (ss)
+                    tcc_error("redefinition of enumerator '%s'",
+                              get_tok_str(v, NULL));
                 next();
                 if (tok == '=') {
                     next();