gpio: pch: add slab include

After change 3ff35cbc
"gpio-pch: Fix Kconfig dependencies"
which enabled COMPILE_TEST as an alternative for the PCH
driver, we get build failures like this:

drivers/gpio/gpio-pch.c: In function 'pch_gpio_probe':
drivers/gpio/gpio-pch.c:359:2: error: implicit declaration
of function 'kzalloc' [-Werror=implicit-function-declaration]
drivers/gpio/gpio-pch.c:359:7: warning: assignment makes
pointer from integer without a cast [enabled by default]
drivers/gpio/gpio-pch.c:442:2: error: implicit declaration
of function 'kfree' [-Werror=implicit-function-declaration]

Fix this by including <linux/slab.h> explicitly.

Cc: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>