Fix build error on MySQL 5.6
Fix build error on MySQL 5.6

    ha_mroonga.cpp:7734:36: error: 'struct KEY' has no member named 'key_parts'

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