selinux: Make selinux_kernfs_init_security static

Fix sparse warning:

security/selinux/hooks.c:3389:5: warning:
 symbol 'selinux_kernfs_init_security' was not declared. Should it be static?

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: Ondrej Mosnacek <omosnace@redhat.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>