RFI in 0.28: Set default cmake build to exclude linearstore on 0.28 release

This change causes the linearstore to be excluded by default in cmake.
This is a trivial change and is identical to the change make on 0.26.

JIRA: https://issues.apache.org/jira/browse/QPID-5654

Reviewboard: https://reviews.apache.org/r/19858/