xen: ifdef CONFIG_HIBERNATE_CALLBACKS xen_*_suspend

xen_hvm_post_suspend, xen_pre_suspend, xen_post_suspend are only used if
CONFIG_HIBERNATE_CALLBACKS is defined, resulting in:

drivers/xen/manage.c:46:13: warning: ‘xen_hvm_post_suspend’ defined but not used [-Wunused-function]
drivers/xen/manage.c:52:13: warning: ‘xen_pre_suspend’ defined but not used [-Wunused-function]
drivers/xen/manage.c:59:13: warning: ‘xen_post_suspend’ defined but not used [-Wunused-function]

If the kernel config is missing CONFIG_HIBERNATE_CALLBACKS.

Simply ifdef CONFIG_HIBERNATE_CALLBACKS the three functions.
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/xen/manage.c

@@ -43,6 +43,7 @@ struct suspend_info {
 	void (*post)(int cancelled);
 };
 
+#ifdef CONFIG_HIBERNATE_CALLBACKS
 static void xen_hvm_post_suspend(int cancelled)
 {
 	xen_arch_hvm_post_suspend(cancelled);
@@ -63,7 +64,6 @@ static void xen_post_suspend(int cancelled)
 	xen_mm_unpin_all();
 }
 
-#ifdef CONFIG_HIBERNATE_CALLBACKS
 static int xen_suspend(void *data)
 {
 	struct suspend_info *si = data;