fdo#68849: Add header guards to all include files

Added header guards to files in directories xml*/*

Change-Id: Ia5dfb9ab494bfbfae7537f2d54ff11331dc8c922
Reviewed-on: https://gerrit.libreoffice.org/9539
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>