s390/hvc_iucv: fix sparse warning

drivers/tty/hvc/hvc_iucv.c:131:25: warning: symbol 'hvc_iucv_get_private' was not
declared. Should it be static?

Signed-off-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>