serial: atmel: add missing dmaengine header

The atmel serial driver uses dmaengine APIs but never included the dmaengine
header as it was getting inculded thru one of driver headers.

commit 3d588f83 - "dmaengine: dw: split
dma-dw.h to platform and private parts" broke this as it moved headers
around.  Fix this by doing the right thing to include the dmaengine header
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Fixes: 08f738be (serial: at91: add tx dma support)
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>

drivers/tty/serial/atmel_serial.c

@@ -37,6 +37,7 @@
 #include <linux/of_device.h>
 #include <linux/of_gpio.h>
 #include <linux/dma-mapping.h>
+#include <linux/dmaengine.h>
 #include <linux/atmel_pdc.h>
 #include <linux/atmel_serial.h>
 #include <linux/uaccess.h>