s390/zcore: remove unneeded linux/miscdevice.h include

drivers/s390/char/zcore.c does not contain any miscdevice so the
inclusion of linux/miscdevice.h is uncessary.

Signed-off-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>