Fix build with mysql-connector-c v8.0.18

[devurandom]

This fixes following compilation failure with version 8.0.18 of mysql-connector-c:

rdf_storage_mysql.c: In function 'librdf_storage_mysql_get_handle':
rdf_storage_mysql.c:450:5: error: unknown type name 'my_bool'; did you mean 'bool'?
  450 |     my_bool value=(context->reconnect) ? 1 : 0;
      |     ^~~~~~~
      |     bool
make[3]: *** [Makefile:886: librdf_storage_mysql_la-rdf_storage_mysql.lo] Error 1

The correct type for calling mysql_options with MYSQL_OPT_RECONNECT in version 8.0 of the library is bool*: https://dev.mysql.com/doc/refman/8.0/en/mysql-options.html

See-Also: https://bugs.gentoo.org/692462

[bodograumann]

I don’t think you can simply replace it like that, because the code must work with mysql-connector-c version 6 and version 8.