dmaengine: remove struct scatterlist for header

Commit 90b44f8 introduces dmaengine_prep_slave_single API which adds
scatterlist.h in dmaengine.h, so defining struct scatterlist is not required

Signed-off-by: Vinod Koul <vinod.koul@intel.com>
Acked-by: Dan Williams <dan.j.williams@intel.com>

include/linux/dmaengine.h

@@ -26,8 +26,6 @@
 #include <linux/dma-direction.h>
 #include <linux/scatterlist.h>
 
-struct scatterlist;
-
 /**
  * typedef dma_cookie_t - an opaque DMA cookie
  *