s390/appldata: add slab.h for kzalloc/kfree

This fixes:
arch/s390/appldata/appldata_mem.c:135:2: error: implicit declaration of function 'kzalloc' [-Werror=implicit-function-declaration]
arch/s390/appldata/appldata_mem.c:141:3: error: implicit declaration of function 'kfree' [-Werror=implicit-function-declaration]

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>