HV: prototyping non-static function

Includes header file of non-static function, and declare the
in-file use function static.

Signed-off-by: Yang, Yu-chu <yu-chu.yang@intel.com>
Acked-by: Anthony Xu <anthony.xu@intel.com>

hypervisor/arch/x86/debug/reboot.c

@@ -3,6 +3,7 @@
  * SPDX-License-Identifier: BSD-3-Clause
  */
 #include <hypervisor.h>
+#include <reboot.h>
 
 int warm_reboot(void)
 {