swiotlb: do not export map_single function

The map_single() function is not defined as static, even though it
doesn't seem to be used anywhere else in the kernel. Make it static to
avoid namespace pollution since this is a rather generic symbol.
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

lib/swiotlb.c

@@ -537,7 +537,8 @@ EXPORT_SYMBOL_GPL(swiotlb_tbl_map_single);
  * Allocates bounce buffer and returns its kernel virtual address.
  */
 
-phys_addr_t map_single(struct device *hwdev, phys_addr_t phys, size_t size,
+static phys_addr_t
+map_single(struct device *hwdev, phys_addr_t phys, size_t size,
 	   enum dma_data_direction dir)
 {
 	dma_addr_t start_dma_addr = phys_to_dma(hwdev, io_tlb_start);