xml2po is deprecated. We've previously replaced xml2po with
itstool in man/generate_translations.mak, but there was still
an instance of it that only is exercised for 'make dist'.
Update that one. Now 'make dist' succeeds on a ubuntu focal
or newer host where xml2po is not available.
Signed-off-by: Serge Hallyn <serge@hallyn.com>
02b200c9aa