Fix an error for cannot initialize return object of type 'enum yytoke…

…ntype' with an rvalue of type 'int'

```
error: cannot initialize return object of type 'enum yytokentype' with an rvalue of type 'int'
    return 0;
           ^
1 error generated.
```

sample/parse.y

@@ -46,7 +46,7 @@ factor          : number
 static enum yytokentype
 yylex(YYSTYPE *lval)
 {
-    return 0;
+    return (enum yytokentype)0;
 }
 
 static void yyerror(YYLTYPE *yylloc, const char *msg)