cmake: rbd resource agent needs to be executable.

cmake: rbd resource agent needs to be executable.

Without this the resource fails in Pacemaker with the error:

Feb 11 12:48:30 mypacemakerhost crmd[5154]:    error: Failed to retrieve meta-data for ocf:ceph:rbd
Feb 11 12:48:30 mypacemakerhost crmd[5154]:   notice: Operation p_rbd_myres_monitor_0: insufficient privileges (node=mypacemakerhost, call=228, rc=4, cib-update=60, confirmed=true)

This worked fine in the jewel version of the package, but is broken in
the luminous one, presumably because of the build changes.

Fixes: http://tracker.ceph.com/issues/22980
Signed-off-by: Tim Bishop <tim@bishnet.net>