arm/xen: make _xen_start_info static

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: NHulk Robot <hulkci@huawei.com>
Signed-off-by: NJason Yan <yanaijie@huawei.com>
Reviewed-by: NStefano Stabellini <sstabellini@kernel.org>
Link: https://lore.kernel.org/r/20200415084853.5808-1-yanaijie@huawei.comSigned-off-by: NJuergen Gross <jgross@suse.com>

arch/arm/xen/enlighten.c

@@ -36,7 +36,7 @@
 
 #include <linux/mm.h>
 
-struct start_info _xen_start_info;
+static struct start_info _xen_start_info;
 struct start_info *xen_start_info = &_xen_start_info;
 EXPORT_SYMBOL(xen_start_info);