ia64: only select ARCH_HAS_DMA_COHERENT_TO_PFN if swiotlb is enabled

Otherwise we get a build failure due in swiotlb-less configs with
non-generic kernels.
Signed-off-by: NChristoph Hellwig <hch@lst.de>

arch/ia64/Kconfig

@@ -28,7 +28,7 @@ config IA64
 	select HAVE_ARCH_TRACEHOOK
 	select HAVE_MEMBLOCK_NODE_MAP
 	select HAVE_VIRT_CPU_ACCOUNTING
-	select ARCH_HAS_DMA_COHERENT_TO_PFN
+	select ARCH_HAS_DMA_COHERENT_TO_PFN if SWIOTLB
 	select ARCH_HAS_SYNC_DMA_FOR_CPU
 	select VIRT_TO_BUS
 	select ARCH_DISCARD_MEMBLOCK