xen: fix backport of previous kexec patch

Fixes the backport of 0b34a166 upstream

Commit 0b34a166 "x86/xen: Support
kexec/kdump in HVM guests by doing a soft reset" has been added to the
4.2-stable tree" needed to correct the CONFIG variable, as
CONFIG_KEXEC_CORE only showed up in 4.3.
Reported-by: David Vrabel <david.vrabel@citrix.com>
Reported-by: Luis Henriques <luis.henriques@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/x86/xen/enlighten.c

@@ -33,7 +33,7 @@
 #include <linux/memblock.h>
 #include <linux/edd.h>
 
-#ifdef CONFIG_KEXEC_CORE
+#ifdef CONFIG_KEXEC
 #include <linux/kexec.h>
 #endif
 
@@ -1848,7 +1848,7 @@ static struct notifier_block xen_hvm_cpu_notifier = {
 	.notifier_call	= xen_hvm_cpu_notify,
 };
 
-#ifdef CONFIG_KEXEC_CORE
+#ifdef CONFIG_KEXEC
 static void xen_hvm_shutdown(void)
 {
 	native_machine_shutdown();
@@ -1879,7 +1879,7 @@ static void __init xen_hvm_guest_init(void)
 	x86_init.irqs.intr_init = xen_init_IRQ;
 	xen_hvm_init_time_ops();
 	xen_hvm_init_mmu_ops();
-#ifdef CONFIG_KEXEC_CORE
+#ifdef CONFIG_KEXEC
 	machine_ops.shutdown = xen_hvm_shutdown;
 	machine_ops.crash_shutdown = xen_hvm_crash_shutdown;
 #endif