videobuf-dma-contig: use gfp_t for GFP flags

Sparse complains if about using unsigned long.
videobuf-dma-contig.c:47:67: warning: restricted gfp_t degrades to integer videobuf-dma-contig.c:47:65: warning: incorrect type in argument 2 (different base types)
videobuf-dma-contig.c:47:65: expected restricted gfp_t [usertype] gfp_mask
videobuf-dma-contig.c:47:65: got unsigned long

index b6b5cc1..9b9a06f 100644
@@ -40,7 +40,7 @@ struct videobuf_dma_contig_memory {
 
 static int __videobuf_dc_alloc(struct device *dev,
 			       struct videobuf_dma_contig_memory *mem,
-			       unsigned long size, unsigned long flags)
+			       unsigned long size, gfp_t flags)
 {
 	mem->size = size;
 	if (mem->cached) {