Build: pacemakerd: properly conditionalize corosync calls

Previously, pacemakerd in its entirety was not built unless corosync support
was enabled, a throwback to when CMAN and Corosync 1 were supported. Now,
pacemakerd is built unconditionally, and just its corosync-related calls are
guarded by corosync support.

This has no effect currently since corosync 2+ is the only supported cluster
layer, but it offers some future-proofing.