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>
3 jobs for annotate-destructor-events in 1 minute and 54 seconds (queued for 1 second)
Status Job ID Name Coverage
  Prep
passed #6190912
debian:buster@container-prep

00:00:20

 
  Build
passed #6190913
build-native-autotools

00:01:34

passed #6190914
build-native-meson

00:00:46