xmerge: fix javadoc errors for JDK 8

Change-Id: Ic8352b7460c36155215d25339650ede40735e65d
Reviewed-on: https://gerrit.libreoffice.org/11189
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>