Ignore host-state backup files

host-state-real.json.bak* kept appearing as untracked files in
git status (the base names were ignored, the .bak rotations were
not). Ignore the rotation files so a future git add does not
accidentally commit local state backups.

.gitignore

@@ -10,3 +10,4 @@ __pycache__/
 *.tmp.*
 host-config.json
 host-config-real.json
+host-state-real.json.bak*