Refactor: include: empty compatibility.h

As of Pacemaker 2.0, the policy for include/crm/compatibility.h will be
that it contains definitions that ease the transition from the previous
major version API.

For example, compatibility.h in Pacemaker 1.1 contained definitions
relevant to transitioning from the Pacemaker 1.0 API; compatibility.h in
Pacemaker 2.0 will remove all those definitions, and replace them with
definitions relevant to transitioning from the Pacemaker 1.1 API.

Thus, developers can include compatibility.h and have the lifetime of the
current major version to update their code to use the latest definitions,
but their code will break if they have not updated by the release of the
next major version.