armv8: fix typo in LINUX_KERNEL_IMAGE_HEADER check

Fixes: 8163faf9 ARMv8: add optional Linux kernel image header
Signed-off-by: NMian Yousaf Kaukab <ykaukab@suse.de>
Reviewed-by: NStephen Warren <swarren@nvidia.com>
Tested-by: NAndreas Färber <afaerber@suse.de>

arch/arm/cpu/armv8/start.S

@@ -18,7 +18,7 @@
 
 .globl	_start
 _start:
-#if defined(LINUX_KERNEL_IMAGE_HEADER)
+#if defined(CONFIG_LINUX_KERNEL_IMAGE_HEADER)
 #include <asm/boot0-linux-kernel-header.h>
 #elif defined(CONFIG_ENABLE_ARM_SOC_BOOT0_HOOK)
 /*