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>
diff --git a/lib/swiotlb.c b/lib/swiotlb.c
index 4abda07..3c365ab 100644
--- a/lib/swiotlb.c
+++ b/lib/swiotlb.c
@@ -537,8 +537,9 @@
  * Allocates bounce buffer and returns its kernel virtual address.
  */
 
-phys_addr_t map_single(struct device *hwdev, phys_addr_t phys, size_t size,
-		       enum dma_data_direction dir)
+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);