finally broken for piped usage

This worked a few months ago so some refactoring has inadvertently caused this. This looks like an easy fix. __closure not fully qualified in finally.hpp. Think a header file might be missing too.

```
.... | repeat_until() | finally(net::Disconnect(socket));
```

Error:
```
In template: use of undeclared identifier '__closure'clang(undeclared_var_use)
finally.hpp(51, 14): Error occurred here
```
