QPID-5528: HA add missing QPID_BROKER_EXTERN declarations.

Missing from previous commit
 r1564010 | QPID-5528: HA Clean up error messages around rolled-back transactions.

git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1564120 13f79535-47bb-0310-9956-ffa450edef68