xwayland: fix xdg_output leak

The xdg_output wasn't cleaned up when destroying the xwl_output.

Signed-off-by: Simon Ser <contact@emersion.fr>
5 jobs for cleanup-xdg-output in 7 minutes and 47 seconds (queued for 13 seconds)
Status Name Job ID Coverage
  Docker Image
passed debian-buster #11516767

00:00:27

 
  Build And Test
passed autotools #11516768

00:07:20

passed meson #11516769

00:03:21

passed meson-noglamor #11516770

00:02:56

passed mingw-cross-build #11516771

00:00:36