Build: schemas: Rebuild versions.rng with pacemaker-$(CIB_max).rng

This ensures that versions.rng gets rebuilt on a schema version bump.
Unfortunately in the current implementation, all pacemaker-*.rng files
get rebuilt when any schema file changes, and so this will happen with
versions.rng too. Nonetheless, it eliminates a manual step when bumping
the schema version.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>