Fix missing annotation for DropSequenceTableChange

Starting with the extension was impossible because of the following exception :
 liquibase.exception.UnexpectedLiquibaseException: No @DatabaseChange annotation for liquibase.ext.sequencetable.change.DropSequenceTableChange
	at liquibase.change.AbstractChange.createChangeMetaData(AbstractChange.java:59) ~[liquibase.jar:na]

This change just add the missing annotation.