Correct build failure for powerpc/pseries builds with CONFIG_SMP not defined.

The function cpu_sibling_mask has no meaning (or definition) when CONFIG_SMP
is not defined. Additionally, the updating of NUMA affinity for a CPU in a UP
system doesn't really make sense.

This patch ifdef's out the code making the affinity updates for PRRN events to
fix the following build break.

arch/powerpc/mm/numa.c: In function ‘stage_topology_update’:
arch/powerpc/mm/numa.c:1535: error: implicit declaration of function ‘cpu_sibling_mask’
arch/powerpc/mm/numa.c:1535: warning: passing argument 3 of ‘cpumask_or’ makes pointer from integer without a cast
make[1]: *** [arch/powerpc/mm/numa.o] Error 1

Signed-off-by: Nathan Fontenot <nfont@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>