x86/xen: make xen_pvmmu_arch_setup() static

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>

arch/x86/xen/setup.c

@@ -985,7 +985,7 @@ void xen_enable_syscall(void)
 #endif /* CONFIG_X86_64 */
 }
 
-void __init xen_pvmmu_arch_setup(void)
+static void __init xen_pvmmu_arch_setup(void)
 {
 	HYPERVISOR_vm_assist(VMASST_CMD_enable, VMASST_TYPE_4gb_segments);
 	HYPERVISOR_vm_assist(VMASST_CMD_enable, VMASST_TYPE_writable_pagetables);