Code review comment for lp:~adeuring/lazr.restful/add-destructor-operation-to-all-decl

When I tried to use @export_destructor_operation() in a Launchpad branch, I noticed the warning

  There were 1 imports of names not appearing in the __all__.
  You should not import export_destructor_operation from lazr.restful.declarations:
      canonical.launchpad.interfaces.hwdb

This is the trivial fix for the wrning.