doc: add missing dependency
doc: add missing dependency

We need .pot to generate .mo.

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