powerpc/rtas: Unexport rtas_online_cpus_mask, rtas_offline_cpus_mask

These aren't used by modular code, nor should they be.

Fixes: 120496ac ("powerpc: Bring all threads online prior to migration/hibernation")
Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190718162214.5694-1-nathanl@linux.ibm.com