QPID-5421: Refactor: clean up QueueObservers - fix CMakelists.txt

Fix error from r1550818 - didn't remove QueueSnapshots.h from CMakeLists.h.

git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1551210 13f79535-47bb-0310-9956-ffa450edef68