doc: re-add "update" target in "doc/locale/${LANG}/LC_MESSAGES"
doc: re-add "update" target in "doc/locale/${LANG}/LC_MESSAGES"

Reported by cosmo0920. Thanks!!!

yoku0825/mroonga,yoku0825/mroonga,naoa/mroonga,mroonga/mroonga,naoa/mroonga,naoa/mroonga,naoa/mroonga,naoa/mroonga,mroonga/mroonga,mroonga/mroonga,yoku0825/mroonga,naoa/mroonga,yoku0825/mroonga,yoku0825/mroonga,mroonga/mroonga,yoku0825/mroonga