When building a random powerpc kernel I hit this build error:

  arch/powerpc/platforms/powernv/opal-imc.c:130:13: error : assignment
  discards « const » qualifier from pointer target type
  [-Werror=discarded-qualifiers]
     l_cpumask = cpumask_of_node(nid);
             ^

This happens because when CONFIG_NUMA=n cpumask_of_node() returns a
const pointer.

This patch simply adds const to l_cpumask to fix this issue.
Signed-off-by: NCorentin Labbe <clabbe.montjoie@gmail.com>
Reviewed-by: NMadhavan Srinivasan <maddy@linux.vnet.ibm.com>
[mpe: Flesh out change log]
Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>