s390/pkey: make pkey_init() static

drivers/s390/crypto/pkey_api.c:1197:12:
  warning: symbol 'pkey_init' was not declared. Should it be static?

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