POWERPC doesn't expect it to be used.

This fixes the linux-next build failure reported by
Stephen Rothwell:

  lib/swiotlb.c: In function 'setup_io_tlb_npages':
  lib/swiotlb.c:114: error: 'swiotlb' undeclared (first use in this function)
Reported-by: NStephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: NFUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Cc: peterz@infradead.org
LKML-Reference: <20091112000258F.fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: NIngo Molnar <mingo@elte.hu>