japanese/kterm: Revert MANPREFIX change in 9a5a5a0

• Just use "install.man" target to install the manual page since ManDirectoryRoot has been updated to point LOCALBASE/share/man on and after xorg-cf-files-1.0.8_1.
• The localized manpage is installed to LOCALBASE/share/man/<lang>.<charset>/ where man(1) looks up, not LOCALBASE/share/man/<lang>/.