Fix unqualified friend declaration
Fix unqualified friend declaration

clang-cl warns about this as a Microsoft extension:
```
async-inl.h(922,16): warning: unqualified friend declaration referring
to type outside of the nearest enclosing namespace is a Microsoft
extension; add a nested name specifier [-Wmicrosoft-unq
ualified-friend]
  friend class WaitScope;
               ^
               ::kj::
```

mologie/capnproto,mologie/capnproto,mologie/capnproto