expat: fix host build issue with docbook

expat: fix host build issue with docbook

Additionally to the fix issued for #6923, we need to disable the docbook
usage also for the host build. This prevents the following error:

checking for docbook2man... docbook2man
configure: error: Your local docbook2man was found to work with SGML rather
  than XML. Please install docbook2X and use variable DOCBOOK_TO_MAN to point
  configure to command docbook2x-man of docbook2X.
  Or use DOCBOOK_TO_MAN="xmlto man --skip-validation" if you have xmlto around.
  You can also configure using --without-docbook if you can do without a man
  page for xmlwf.

Signed-off-by: Michael Heimpold <michael.heimpold@i2se.com>