swiotlb: mark is_swiotlb_buffer static

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

kernel/dma/swiotlb.c

@@ -429,7 +429,7 @@ void __init swiotlb_exit(void)
 	max_segment = 0;
 }
 
-int is_swiotlb_buffer(phys_addr_t paddr)
+static int is_swiotlb_buffer(phys_addr_t paddr)
 {
 	return paddr >= io_tlb_start && paddr < io_tlb_end;
 }