mariadb10.0.4: suppress a warning

    ha_mroonga.cpp: In function ‘const char* mrn_inspect_extra_function(ha_extra_function)’:
    ha_mroonga.cpp:258:10: warning: enumeration value ‘HA_EXTRA_EXPORT’ not handled in switch [-Wswitch]
       switch (operation) {
              ^

HA_EXTRA_EXPORT is backported to MariaDB from MySQL since MariaDB
10.0.4. Now, mroonga can't build with MariaDB 10.0.{1,2,3}.