configure: ENABLE_NPAPI_FROM_BROWSER requires Xt library

Change-Id: I023927897787ae046baf2751d0e56bfca3fb688f
(cherry picked from commit 1366a41c320a9bb173dc5e066ab532d679d89bcc)
Reviewed-on: https://gerrit.libreoffice.org/7346
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>