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 8 seconds (queued for 13 seconds)
Status Name Job ID Coverage
  Docker Image
passed debian-buster #12712251

00:00:31

 
  Build And Test
passed autotools #12712252

00:06:36

passed meson #12712253

00:02:35

passed meson-noglamor #12712254

00:01:57

passed mingw-cross-build #12712255

00:00:47