fix: create foreign key constraint when tablePerHierarchy=false (#857)

grails-datastore-gorm-hibernate5/src/main/groovy/org/grails/orm/hibernate/cfg/GrailsDomainBinder.java

@@ -1615,6 +1615,7 @@ protected void bindJoinedSubClass(HibernatePersistentEntity sub, JoinedSubclass
         bindSimpleValue(identifier.getType().getName(), key, false, columnName, mappings);
 
         joinedSubclass.createPrimaryKey();
+        joinedSubclass.createForeignKey();
 
         // properties
         createClassProperties(sub, joinedSubclass, mappings, sessionFactoryBeanName);