xmerge: static non-final variable in the initialization

Change-Id: Ide7ea42e570100766ef4d97fc3c29c5a5028ce37
Reviewed-on: https://gerrit.libreoffice.org/10662
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>