powerpc: Exclude arch_sd_sibiling_asym_packing() on UP

Only SMP systems care about load-balance features, plus this
saves some .text space on UP and also fixes the build.

Reported-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Michael Neuling <mikey@neuling.org>
LKML-Reference: <tip-76cbd8a8@git.kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>