use whitespace to define subsignature for unpacking

see rakudo/rakudo@dd5687cc9fb96ce175799090675fcb6bdc3dd067

Example of the error output:
```
===SORRY!===
Shape declaration with () is reserved;
  please use whitespace if you meant a subsignature for unpacking,
  or use the :() form if you meant to add signature info to the function's type
at t/08-callbacks.t:20
------> sub TakeACallback(&cb⏏()) is native('./08-callbacks') { * }
```