spi/dw_spi: Fix missing header

Currently, build on PPC dies with:
In file included from drivers/spi/dw_spi_mmio.c:16:
include/linux/spi/dw_spi.h:147: error: field ‘tx_sgl’ has incomplete type
include/linux/spi/dw_spi.h:149: error: field ‘rx_sgl’ has incomplete type

Add linux/scatterlist.h include to dw_spi.h, because we need to know
the contents of the structure.
Signed-off-by: NJiri Slaby <jslaby@suse.cz>
Signed-off-by: NGrant Likely <grant.likely@secretlab.ca>

include/linux/spi/dw_spi.h

@@ -2,6 +2,7 @@
 #define DW_SPI_HEADER_H
 
 #include <linux/io.h>
+#include <linux/scatterlist.h>
 
 /* Bit fields in CTRLR0 */
 #define SPI_DFS_OFFSET			0