swiotlb: Make swiotlb_{alloc,free}_buffer depend on CONFIG_DMA_DIRECT_OPS

Otherwise this causes unused symbol warnings for configs that build
swiotlb.c only for use by xen-swiotlb.c and that don't otherwise select
CONFIG_DMA_DIRECT_OPS, which is possible on arm.

Fixes: 16e73adb

 ("dma/swiotlb: Remove swiotlb_{alloc,free}_coherent()")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: iommu@lists.linux-foundation.org
Cc: konrad.wilk@oracle.com
Link: https://lkml.kernel.org/r/20180323174930.17767-1-hch@lst.de