x86: add proper header for reboot_force

This patch fixes one sparse warning by including the appropriate
header for the reboot_force symbol.
Signed-off-by: Dmitri Vorobiev <dmitri.vorobiev@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/kernel/reboot.c

@@ -9,6 +9,7 @@
 #include <asm/desc.h>
 #include <asm/hpet.h>
 #include <asm/pgtable.h>
+#include <asm/proto.h>
 #include <asm/reboot_fixups.h>
 #include <asm/reboot.h>