kj/async-unix-test: fix -Wmissing-field-initializers in UrgentObserver
kj/async-unix-test: fix -Wmissing-field-initializers in UrgentObserver

Warned by gcc 5.3.1 and clang 3.7.0

mrdomino/capnproto,jparyani/capnproto,hntrmrrs/capnproto,mrdomino/capnproto,jparyani/capnproto,mrdomino/capnproto,hntrmrrs/capnproto,pjulien/capnproto,jparyani/capnproto,mologie/capnproto,hntrmrrs/capnproto,mologie/capnproto,mologie/capnproto,pjulien/capnproto,pjulien/capnproto