Update struct.sql

Error Code: 1067. Invalid default value for 'dt_access_date'
Resolve: http://dev.mysql.com/doc/refman/5.7/en/timestamp-initialization.html
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Use of DEFAULT CURRENT_TIMESTAMP and ON UPDATE CURRENT_TIMESTAMP is specific to TIMESTAMP and DATETIME. The DEFAULT clause also can be used to specify a constant (nonautomatic) default value; for example, DEFAULT 0 or DEFAULT '2000-01-01 00:00:00'.

Note
The following examples that use DEFAULT 0 do not work if the SQL mode is set to cause “zero” date values (specified, for example, as 0 '0000-00-00 00:00:00') to be rejected. Such dates are rejected in strict SQL mode (as of MySQL 5.7.4) or the NO_ZERO_DATE mode is enabled (before MySQL 5.7.4). Be aware that the TRADITIONAL SQL mode includes “zero” date rejection.