luci-app-statistics: stat-genconfig: add missing i18n module import

Fixes the following runtime error:

    /usr/bin/lua: /usr/bin/stat-genconfig:289: attempt to index field 'i18n' (a nil value)
    stack traceback:
        /usr/bin/stat-genconfig:289: in main chunk

Reported-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>
Signed-off-by: Jo-Philipp Wich <jo@mein.io>