async_tx: use of kzalloc/kfree requires the include of slab.h

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/dma/timb_dma.c

@@ -27,6 +27,7 @@
 #include <linux/io.h>
 #include <linux/module.h>
 #include <linux/platform_device.h>
+#include <linux/slab.h>
 
 #include <linux/timb_dma.h>