Comparison between signed and unsigned integer expressions

**$ make all**

```
....
util.cc: In member function ‘bool hpenc::HPEncHeader::toFd(int, bool)’:
util.cc:334:47: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   return (::write(fd, out.data(), out.size()) == out.size());
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~
....
```
