zephyr: Update include path to reboot.h.

The reboot header was moved to a different path in Zephyr v2.6.0. The
old path was deprecated for two releases (v2.6.0 and v2.7.0) and will no
longer be supported after Zephyr v2.7.0.
Signed-off-by: Maureen Helm <maureen.helm@intel.com>

ports/zephyr/modmachine.c

@@ -28,7 +28,7 @@
 
 #include <stdint.h>
 #include <stdio.h>
-#include <power/reboot.h>
+#include <sys/reboot.h>
 
 #include "py/obj.h"
 #include "py/runtime.h"