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: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>