Fix the following sparse warning:

arch/x86/xen/setup.c:998:12: warning: symbol 'xen_pvmmu_arch_setup' was not
declared. Should it be static?
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20200408024605.42394-1-yanaijie@huawei.comSigned-off-by: Juergen Gross <jgross@suse.com>