swiotlb: unexport is_swiotlb_active

Drivers have no business looking at dma-mapping or swiotlb internals.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Juergen Gross <jgross@suse.com>

kernel/dma/swiotlb.c

@@ -956,7 +956,6 @@ bool is_swiotlb_active(struct device *dev)
 
 	return mem && mem->nslabs;
 }
-EXPORT_SYMBOL_GPL(is_swiotlb_active);
 
 #ifdef CONFIG_DEBUG_FS