MYSQL_PLUGIN_IMPORT did not work correctly for the Rocksdb helper library
rocksdb_aux_lib, because that library was not compiled with
-DMYSQL_DYNAMIC_PLUGIN.

Fix dbug such that it does not depend on exported data, only on functions
(which do not need MYSQL_PLUGIN_IMPORT decoration)

Use a "getter" function _db_my_assert() instead of DLL-exported variable.