doc: Japanese man pages is not generated when docbook-utils is used

Character encoding of Japanese man pages is UTF-8. But docbook-utils
can't treat it (and don't have --encoding option that use in
Makefile). So change to Japanese man pages is not generated when
docbook-utils is used.

Signed-off-by: KATOH Yasufumi <karma@jazz.email.ne.jp>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>