ptp_pch: Add missing #include <linux/slab.h>

drivers/ptp/ptp_pch.c: In function 'pch_remove':
drivers/ptp/ptp_pch.c:576:2: error: implicit declaration of function 'kfree' [-Werror=implicit-function-declaration]
drivers/ptp/ptp_pch.c: In function 'pch_probe':
drivers/ptp/ptp_pch.c:587:2: error: implicit declaration of function 'kzalloc' [-Werror=implicit-function-declaration]

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>