staging: ipack: add missing include (implicit declaration of function 'kfree')

On ARCH=alpha make allmodconfig:

linux-2.6/drivers/staging/ipack/bridges/tpci200.c: In function 'tpci200_free_irq':
linux-2.6/drivers/staging/ipack/bridges/tpci200.c:188:2: error: implicit declaration of function 'kfree' [-Werror=implicit-function-declaration]
linux-2.6/drivers/staging/ipack/bridges/tpci200.c: In function 'tpci200_request_irq':
linux-2.6/drivers/staging/ipack/bridges/tpci200.c:215:2: error: implicit declaration of function 'kzalloc' [-Werror=implicit-function-declaration]

Fixed by adding <linux/slab.h> header

CC: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
CC: Samuel Iglesias Gonsalvez <siglesias@igalia.com>
CC: Jens Taprogge <jens.taprogge@taprogge.org>
CC: "Miguel Gómez" <magomez@igalia.com>
CC: devel@driverdev.osuosl.org
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>