protocol: Specify wl_callback::done to be a destructor event

It's the only destructor event in the core protocol, and destructor
events were previously unannotated.

Signed-off-by: Tadeo Kondrak <me@tadeo.ca>
Status Job ID Name Coverage
  Prep
passed #3067459
debian:buster@container-prep

00:00:47

 
  Build
passed #3067460
build-native-autotools

00:01:01

passed #3067461
build-native-meson

00:00:24