Index: kernel/arch/amd64/include/boot/boot.h
===================================================================
--- kernel/arch/amd64/include/boot/boot.h	(revision df4ed852a2d1b242f9bdce0a873009a2cb77cec7)
+++ kernel/arch/amd64/include/boot/boot.h	(revision 2d03471012eecc600496d9c6cdf98367c17d7784)
@@ -43,5 +43,14 @@
 #define MULTIBOOT_HEADER_FLAGS	0x00010003
 
-#define MULTIBOOT_LOADER_MAGIC	0x2BADB002
+#ifndef __ASM__
+
+#ifdef CONFIG_SMP
+
+/* This is only a symbol so the type is dummy. Obtain the value using &. */
+extern int _hardcoded_unmapped_size;
+
+#endif /* CONFIG_SMP */
+
+#endif /* __ASM__ */
 
 #endif