Fix the following sparse warning:

arch/arm64/xen/../../arm/xen/enlighten.c:39:19: warning: symbol
'_xen_start_info' was not declared. Should it be static?
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Link: https://lore.kernel.org/r/20200415084853.5808-1-yanaijie@huawei.comSigned-off-by: Juergen Gross <jgross@suse.com>