cmake: use "mroonga" instead of "ha_mroonga" for plugin name
cmake: use "mroonga" instead of "ha_mroonga" for plugin name

GitHub: fix #30

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