Deprecation warning with tempnam

```
/.../mruby/build/mrbgems/mruby-socket/test/sockettest.c:8:32: warning: 'tempnam' is deprecated: This function is provided for compatibility reasons
      only. Due to security concerns inherent in the design of tempnam(3), it is highly recommended that you use mkstemp(3) instead. [-Wdeprecated-declarations]
```
