Missing compile option `-lrt` when target is Linux

If not set, j2v8 may throw `undefined reference to 'clock_gettime'` at runtime

I added this option to https://github.com/eclipsesource/J2V8/blob/master/cmake/NodeJsUtils.cmake#L142 and solved this issue

```cmake
set (njs_${config_name}_libs ${njs_libs} -lrt PARENT_SCOPE)
```