Low: fix missing qbarray.h include, no longer brought by qbipc[cs].h

Pacemaker should by no mean rely on fancy (and sometimes improper)
transitive includes present with some versions of libqb, and use
the API properly; here: when you use functionality from qbarray.h,
care to include it.

References:

http://oss.clusterlabs.org/pipermail/users/2016-October/004411.html
(initial report, thanks Ferdinando!)
https://github.com/ClusterLabs/libqb/pull/228/commits/c1c26c94cb38e1b40d063a7e5fa29f2de033ed37
(commit that introduced the change at the libqb side)