ntpstats-metrics.rb: Fix uninitialized constant NtpStatsMetrics::Socket

With the embedded ruby, ntpstats-metrics.rb fails due to a `NameError` like this:

```
Sensu::Plugin::CLI: ["Not implemented! You should override Sensu::Plugin::CLI#run."]
./plugins/system/ntpstats-metrics.rb:26:in `<class:NtpStatsMetrics>': uninitialized constant NtpStatsMetrics::So
cket (NameError)
        from ./plugins/system/ntpstats-metrics.rb:14:in `<main>'
```

This patch fixes the issue by requiring `socket` module explicitly.