fix access denied error at incremental build

On windows (both MSVC and mingw), incremental build was failed like this.
```
mruby\src\array.c : fatal error C1083: コンパイラの生成した ファイルを開けません。'build\host\src\array.obj': Permission denied
rake aborted!
```
I think the output file is locked by File object.
This change works for me.Is this correct?