s390/cio: make vfio_ccw_io_region static

Fix the following sparse warning:
drivers/s390/cio/vfio_ccw_drv.c:25:19: warning: symbol 'vfio_ccw_io_region'
was not declared. Should it be static?

Signed-off-by: Sebastian Ott <sebott@linux.ibm.com>
Message-Id: <alpine.LFD.2.21.1810151328570.1636@schleppi.aag-de.ibmmobiledemo.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>