make use FFI::Sweet qw( :all ) work

I noticed that `use FFI::Sweet qw( :all )` gives an error:

```
kraken% perl -E 'use FFI::Sweet qw( :all )'
Could not find sub '_void' to export in package 'FFI::Sweet' at -e line 1.
BEGIN failed--compilation aborted at -e line 1.
```

This patch fixes that.