xquartz: Ensure that NSRunAlertPanel() is run on the main thread

Fixes: https://github.com/XQuartz/XQuartz/issues/30
Signed-off-by: Jeremy Huddleston Sequoia <jeremyhu@apple.com>
4 jobs for xquartz-nsalert-crash-fix in 4 minutes and 47 seconds (queued for 3 seconds)