vfio/mdev: add static modifier to add_mdev_supported_type

Set add_mdev_supported_type as static since it is only used within
mdev_sysfs.c.  This fixes -Wmissing-prototypes gcc warning.

Signed-off-by: Paolo Cretaro <paolocretaro@gmail.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>