dmaengine: pch: fix compilation for alpha target

commit 4828b493 introduced COMPILE_TEST for this driver and this cause compile
failure on alpha as kzalloc wasnt availble for this arch in included header, so
explictly add slab.h
Reported-by: Nkbuild test robot <fengguang.wu@intel.com>
Signed-off-by: NVinod Koul <vinod.koul@intel.com>

drivers/dma/pch_dma.c

@@ -21,6 +21,7 @@
 #include <linux/dma-mapping.h>
 #include <linux/init.h>
 #include <linux/pci.h>
+#include <linux/slab.h>
 #include <linux/interrupt.h>
 #include <linux/module.h>
 #include <linux/pch_dma.h>